Most of the UTH's are made to go on the outside of the cage. Here is a list

OUTSIDE:
Zoo-Med
Flexwatt
Exo-Terra
Cobra

INSIDE:
Cobra
Kane
Stanfield

You can use any UTH with wood depending on the thickness of the wood but as a general rule using them with glass, plastic or inside the cage covered with plexiglass(for the outside models) is best..
-Matt