wondering what people were using to plug light leaks as I am in that stage now

I used foam.

foam like weather stripping foam?

I want to plug all the cracks and crevices and etc, but I guess I could do that with foam too... hmmm...

Box wise I sealed all my joints etc with bondo ( that I sanded down cleanly before I painted) on any seam that would not be opened again, and my lid sits on thin track of wood the lenght of the unit that has foam stripping on it that the lid sits on to seal it up. I also have that striping on the front and back top seam of the box that my lid seats on. And my unit is sealed up very nicely. Oh and my foam is just a thin black foam sheet from a craft store that I cut to size and glued down.

I used just about everything mentioned above.

Elmer's wood filler, Caulk, weatherstripping.

In addition, I used those locking mechanisms you see on those trunks people use when they go to summer camp that have a 2 part latch. When you close it, it pulls down the top to better seal the enclosure.

I used black caulking and weatherstripping

I just built my box so that the parts fit together.

Well, okay, a bit of weatherstripping foam on one side...