Mount it on the left inside of the cabinet, on the support wall. Dont worry about moisture, it aint like your hanging it in a rainforrest!! It will be fine, the coils are well sealed. If your worried about the heat of the ballast, dont, it wont get hot enough to burn wood. But, you could put a 'larger than the ballast' piece of aluminum flashing between the ballast and the wood to act as a heat dissipator. Lowes/Home Dep sell aluminum flashing in rolls real cheap $5-10, and you can cut it to fit. If your ballast doesnt have any place to mount screws through, buy some metal strapping (1/2"Wx2'L the type with holes in it, real cheap $2) and use that to strap around the ballast.