I've built abut 20 racks in the past year, consisting of at least 5 different designs. Here's some thoughts I have about some of your points/concerns.

Firstly, melamine is a wood based product that is thermally fused to melamine resin, which makes a strong and waterproof material. It is HEAVY. It also will not warp over time like you would see in almost any kind of plywood product. Warping is associated with fluctuations in humidity within the wood structure over time. This is often exacerbated by the natural grains and imperfections within the natural wood structure found in plywood. Bottom line: Melamine is heavy as hell, but is also waterproof and will be more resistant to warping. Its not toxic to snakes either. I think if you used plywood, the product will warp over time -- without question. Just depends on the rate and frequency of humidity fluctuations.

I like the implementation of Expanded Foam PVC (XPVC) in snake racks. While it is heavier than plywood, it is resistant to warping. Its also waterproof, doesn't chip like melamine, and looks more attractive in my opinion. Its more expensive than melamine products, but it will last much longer. Because they're lighter, they can be stacked to save valuable square footage. However, XPVC must be purchased from a plastics distributer, and hopefully from on that will cut the sheets to specifications on your behalf for a decent price. Not everyone has a plastics distributor in their city, so some people are deterred by the need to travel for supplies.

After building so many racks, I will definitely be using only XPVC from now on. When weighing the pros and cons, there is no question -- in my opinion of course.