good work, looks great
some plywoods can block heat from heat mats, i've never used plywood thick enough to come across that issue myself, but it is an issue people come up against; RHP would be the best option in that case - unless you wanted to add a false floor to install the heat mat inside and seal it up, obviously this is more work if the heat mat fails
If you already have a heat mat, then its worth testing it before investing in an RHP which are generally 4-5 times the price (in the UK at least, but we dont have that many manufacturers of them here).