[Uhehner]

 I mounted a 1/2" thick X 4" wide teak board in the overhead after I located all the blocks in the area. You can feel the blocks of wood with your hands by pressing on the headliner.  When you drill through the headliner material you will hit the first layer of hard fiberglass covering the blocks. Continue drilling. Then you will feel the soft wood of the block. Then you will feel the hard layer of glass that is the cabin top. DO NOT continue drilling into the cabin top. You can now determine the thickness of the blocks. Add for the board you are installing and subtract about 1/4" to be safe. That should do it. Good Luck.  Capt. George