You could line the compartments, sure. Melamine is very hard to cut accurately- I tried, lol; have it cut by a pro. Aquarium silicone seal is good too. The open sides- if you want air-flow (& I would), you could get some perforated metal from home improvement store...sold by sheets (36" x 36" I think?), it's easy to cut & fit with metal shears, & you can nail it to the wood easily. Be sure you have the smooth side INSIDE & the slightly abrasive side out, since most snakes will be sniffing the outside air thru the holes & you don't want them to scrape their nose raw. Perf. metal is sold for screen door replacements etc.- I used it & liked it a lot on my cabinet conversion to snake home, & in the winter, I reduced air-flow by putting thin wood panels over the outside...easy!
That heat pad looks like it will work just fine, though they don't tell you what temp. it reaches? Ask! & you still need to control it by thermostat, but you know that.I think it's fun to re-purpose cabinets for snakes, sounds like you've given it some good thoughts already, & there's more than one way to do things.
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