ive just asked a pal ov mine same thing, he has done it for years, put a heat mat in the bottom ov box and cover it with very thin ply wood then put whatever bedding you use on top, plywood stops dog from scratching and puncturing mat, he said they cost 1p per hour, may sound xpensive but well worth it for good dogs, mats come in differant sizes, dont know prises for them yet.