Sealing small cabinets

It's not that the wood is cracked on the back, just the edges.... say you have a rectangle, and you put the piece of wood on the back, nail it down in a few places... though you can still see it's not a super tight seal, and if you look inside you can see a little light around the edges.

[FrequenC]

Personally, if the back piece can be removed easily enough I'd replace it entirely and use an adhesive to create a tighter seal. However, like I said it's all up to how much you're willing to spend. If you just want to seal up what's on there, then you can use a thin bead of something like clear GE silicone II caulk.