OK, there are several points I want to make in response to what you've said.
1) The cage she's in now should be fine.
a. 20 gallon is plenty big, and plenty small enough for her to feel secure.
b. Does "tilted the cage on its side" mean that you have a screen-top 20-gallon-long tank that is meant for reptiles (a "Critter Cage"?), and that the top has now become the "front"?
c. You said you have a UTH AND a ceramic heat emitter at the same time, perhaps plugged into the same thermostat. Use ONE OR THE OTHER! It will be better for you, your electric bill, and your snake. If your current thermostat is not up to the task, get a better one. If your UTH is not up to maintaining temps in the 80's (NOT THE 90's!!!!!) then it is probably too small for what you are trying to do - get one that is appropriately sized and powered.
d. If you have a ceramic heat emitter on top of glass, sooner or later the GLASS will CRACK! I love ceramic heat emitters, eventhough they dry humidity away like nothing else I know of... I have been thinking of getting some just to boost my other temp control stuff in the winter (since my husband likes to fiddle with the house thermostat when I am not looking) - but they are very good at making heat, so be sure that you are not setting yourself up for a major fire hazard as well!
e. You said you have a divider in your 20 gallon cage now. WHY? How much space is that little snake getting? A 20 gallon cage is perfect! Smaller than that, and I am concerned that you don't have enough space to properly keep a warm end and cool end so that your snake can effectively thermoregulate herself. I can only assume you are doing this to try to help her feel secure. If she has enough places to hide (like one on the warm end and one on the cool end), she doesn't need anything else. I like to use about 2-3 inches of Spanish moss throughout the cage, on top of newspaper substrate for my Dum, as she can hide underneath the moss or burrow through it very easily. This amounts to one bag of Spanish moss as I buy it from the local home improvement store. I just freeze a new bag of moss for about a week before I use it, just in case there are any bugs in the bag. The CAGE shouldn't have to be covered 24/7 to help your snake feel secure either!!
2) Amy (reptilicus81) is right - your set up is incredibly complex, and I do not see the need for it, regardless of your house's lack of insulation - or TX weather. My house has CRAP for insulation; I'm only in central IL - but have lived in NC among many uprooted Texans (they tell me the temp flux in NC is pretty close to TX) - and yes, my house temperature here fluctuates a lot! So does nature!! These animals don't live inside a bubble in the wild. The only thing you need to ensure is that the fluctuations stay within an acceptable range; you don't have to master the environment to the hundredth of a degree to keep a healthy Dumeril's Boa. Ideally strive for no higher than 85, and no lower than 70 degrees Fahrenheit. Fluctuation is okay when you have a temp gradient set up within the cage so your snake can thermoregulate and seek out the area that is most comfortable for her.
3) Snakes are in general no where near as complex as you are making this. I commented before that you seemed to be approaching space for this little girl like you should for a lizard. I DID understand in the first place that you were talking about building in dividers to expand the space out over time - but I think that is going overboard & beyond anything that is neccessary. I think also that if you enjoy this kind of complexity you should get into salt water fish - THEY need that kind of madness. Snakes are WAAAAAYYYY simple.
4) Humidity. What % humidity is it that you are striving for??? No snake actually needs to be kept WET - not even Blood Pythons, contrary to popular belief - and too much humidity can cause a TON of health issues!! Especially with your cool damp air from your tricked-out humidifier, I am most concerned about what you are doing with humidity. Most of the available care sheets that I have seen on Dum's call for a TON of humidity... and it really isn't necessary! There is a lot of misinformation that gets circulated about this species (and I hope that I am not behind the spread of any of it myself!). Amy told you she mists once a week. I mist a little once or twice daily because my cage is not very good at holding humidity - but misting with a hand-held spray bottle accomplishes all that my Dum actually needs. (I suggest you try this, just make sure that you use a spray bottle that has NEVER had any chemical cleaners inside of it!!!)
5) Two tools I strongly suggest: (1) an infrared heat sensing gun, and (2) a good quality, DIGITAL thermometer/hygrometer. For a heat gun, I have (and love) the PE2 model from ProExotics [ http://www.tempgun.com/main.html ] - the most important thing about a heat gun is that is senses the SURFACE TEMP. For a good digital thermometer/hygrometer, there are several options available. I have a small little one that Fluker's makes. I have also seen many people use indoor/outdoor digital weather gauges/stations (don't really know what to call them) from Radioshack or a similar electronics store. The great thing about those weather gauges is that they give a read out of temp & humidity for TWO locations simultaneously (think warm end & cool end of your cage).
6) You didn't answer my primary question: why are you convinced you need to make these changes? Did you read a lot of this in some online care sheet? Did the vet make you feel guilty over your snake becoming ill? You said back when you were getting ready for the snake to arrive from the breeder that you had done your homework on this species. Did the breeder/supplier/seller tell you to go through all of this?
I really want to help, and I sincerely hope I am not becoming offensive. I am feeling a bit frustrated - and what is probably getting me the most is that a lot of people won't summon the kind of energy necessary to care for a new pet. You've done so much more than that, and I don't like to see you waste your efforts! Best of luck to you.
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)