What I have done with wooden enclosures is ................
Cut a hole in the wood floor, that is a little bigger than your heat pad. Then cut a piece of plexiglass,
that is the size of the whole floor of the enclosure. Put the plexiglass on the floor, and use some
silicone to seal all around the edges. Then you are able to stick your heat pad, and therm. probe on the bottom
of the plexiglass, and it will give you your required hot spot.