My GoBox is cobbled out of whatever I had laying around mostly. I use a roll-up J-pole I made from 450 Ohm ladder line for the conveneince of being portable, and it can be stored inside the GoBox. I use a more elaborate copper J-pole when I have my truck.
The only recommendation here added would be a couple of 6" wide reels of waterproof adhesive tape. Do "poor man's lamination" of the box, with an inch of overlap for every pass of tape.
