I've built about every type of rack imaginable. I've even built extremely complex racks for turtles, chamelons, and diurnal lizards that required screen tops, lighting, misting systems, etc. I've also built racks that were equally simple.
I'll post three decent links below. You'll have to cut and paste them into your browser.
First of all, the gap for hognose snakes (except for possibly Lystrophis - I've not worked with those) does not have to be very tight. Of the species you have listed below, the kingsnake and the womas (if the womas are still hatchlings) will be the best escape artists. But I gather this rack is going to be geared towards a group of North American Hognose (Heterodon spp.) and not the misc. other snakes in your collection.
I could go on and on about building racks, but I have a few simple tips.
1) Racks can be built sturdily by only using simple edge joints. In other words, dado or rabbet joints are not absolutely necessary. If anything they confuse the measurement calculations of the average rack builder. I write this, of course, assuming you'll be using plywood, melamine or some combination of the above for your rack. If using plastic there is some merit to utilizing a structural joint.
2) Use two melamine or plywood spacers to set the gap between your tiers. I believe this is best shown in the finegtps link listed below.
In another link the box itself is shown being used as a spacer. This is not a great idea from my experience.
3) Always err on the side of too much gap rather than too little space. It's easier to fill in a gap with thin sheets of material, or to leave that space for under tank heaters, etc, vs. having to add space later. This is by far the biggest mistake novice rack builders make. Many keepers build a rack, move it into a warm herp room only to find that the heat of the room drys out the material and causes it to shrink. I've actually heard of boxes becoming seized in the tiers and had to be forced out.
As far as heating, I've always avoided offering advice in this area due to the potential for disaster. Besides, many here are more experienced with flexwatt and are better able to offer advice.
Lastly, I'll attach a picture of a rack I built for $40. I used melamine bull-nosed shelving from Menards for the shelves. The sides are cheap, 1/2" plywood. In this picture I was only using the rack to hold supplies and mealworms so it did not have a back. When I did have a back on it I simply cut a piece of foil-covered styrene insulation and attached in it place.
For heat I always used Cobra UTH pads. The look like heat tape this is already pre-wired. It is my understanding these are safer than Flex-Watt, although more expensive.
http://www.finegtps.com/Racks.htm
http://www.arbreptiles.com/cages/rack.html
http://www.kingsnake.com/obsoleta/projects.htm
