You have the right idea but I guess it depends on how big a tile you want to make. And what sort of enclosure you have; if it's wooden then a heat cable under a tile is the go, if it's glass then you could buy a heat mat and stick it underneath the glass.
I made 1 for my Lace Monitors in their outdoor enclosure that was 2 tiles glued together with a spiralled cable glued in and a wooden frame around the edge to keep it solid
Looking at going with a wooden enclosure but as I am going to use form ply I may have to scrap my original idea and go with your design. Hard to get anything to stick to form ply. Your "sandwich" design would be easier. Cheers. Much appreciated.
Form ply is very heavy ( and expensive unless you get freebies). I built my wooden ones out of 42mm DAR pine and dressed it in shiplap with a 7 ply base and then stained it. A lot lighter, you can even get pine air vents to keep it natural looking