well there is such a thing as a mat stat for heatmats, they should be fine under plastic. i do think its generally accepted that any uth goes under the cage and not inside it, with wooden enclosures you can put a mat inside as long as it is statted and monitored carefully, but if you dont need to then dont bother lol, with glass and plastic the mats work just as well underneath so theres no need to put them in with the snakes, its just safer.