My last box I made, I used some old 3/8ths tubing from a proto type hydro set I was playing with. I just tacked inside of the opening. I'll try to post up a pic....


Then I added a strip of 3/4" sticky backed insulation, thats at any hardware store, around the face where the door will close on to it.

I try a paint black where ever I'm trying to stop light. There are many ways, but framing out as someone said is always the best IMHO.