>>Have any of you ever used any of the pvc or cdpp (controlled density polypropylene) sheets to build your cages? If so, where did you purchase the sheets & what did it cost?
I have not actually made the step to building entire cages from these products. I have, however, done a fair amount of exerimenting with these materials and feel like I could build a cage if I wanted to.
First, if weight is a concern look no further than 1/4" expanded PVC. If you have plastic distributors in your area you should be able to get it for less than $50 per sheet.
Other plastics are just remarkably heavy. Much heavier than most people suspect. In fact many have a greater density that plywood. Granted you can often use a thinner product but you have to weigh your tradeoffs. For me those tradeoffs are cost and weight. Looks are also a concern but I'd gladly give them up for a cheap and light cage.
If you have a thermal welder then polypropylene and polyethylene are worth looking at. A bit heaver, but the 1/4" stuff is very durable.
Regardless, I just don't have enough information to suggest to you how to build a 6' cage with it.
Typically I would suggest thermal forming a sheet to avoid joints. This is how just about everyone does it. But for a 6' long cage you need to bend around the width, not the length. Since your cages will be 6x2x2 this means you need a width of 6.5' (assuming you used 3" lips).
You could use the plastic angle to edge-join PVCX but I just can't say how rigid a 6' long cage would be.
>>That being said, i'm also considering plywood, especially now that i've seen how nice they can be. I saw some very nice looking cages built by Chris Harper. I want to make them 6' x 2' x 2'. Do you think I will have any problems with bowing?
Bowing is easy to avoid with plywood. I would recommend fairly tall upper and lower lips as well as a center support than sits *behind* these lips.
I still believe plywood is a great choice. The 1/2" stuff is not that much heaver than many plastics, it's cheap, easy to work with, and you can seal it however you like.
Remember you can build a 900 gallon aqarium with 3/4" plywood and epoxy and have it hold water for a lifetime. That's durable enough for me 
But that also brings up the major downfall to plywood - sealing it. The durable, last-a-lifetime products are hard to work with and/or take a very long time to offgas. The easier to work with products often need to be reapplied every few years.
With plastics, weld it and forget it.
There were some posts from somebody regarding 1/2" plywood cages for Olive Pythons. I wish I remembered the guys name or how long back in the archives to look. They were very helpful posts.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)