I use RHP's in most/all of my melamine and PVC enclosures - about 30 total - and have had no issues with them. I would use one over a DHP/CHE in this application.

There are heat mats designed specifically for use inside the enclosure; they are sealed units that can get wet. I think UltraTherm is one, but I've never used that brand. Big Apple Herp sells a brand called IntelliTemp and I have a few of those. I use a dimmer switch/rheostat with them to create a basking spot since those enclosures also have an RHP that keeps the ambient temp constant, so I don't have to worry about the heat mat temperature swinging up and down with the ambient room temperature because it's regulated by a dimmer/rheostat rather than a thermostat.