I cut the aluminum divider out of a 4x5 film holder. Made a 3 x 4 plate holder out of a manila folder & layered up 4 ply mat board. The manila folder layer resides in the film slot, with the mat board layers built up behind ( 2 layers 4ply seems just shy of 2.5mm ). Your right that it is too thin, so I built a box out of 2 layers of foam core painted with black primer. The whole thing is pretty close to 2" in thickness. I need to stop off at the hardware store & get Krylon Ultra Ultra Flat Black. I guess that is what a lot of the DIY telescope builders use for anti reflective coating. I'll PM some photos. Bjelkhagen used a modified graphmatic film holder, which is pretty deep. My polaroid holder is not deep enough and is made of pressed steel.
The GP-2 is considered a solution-physical developer because of the thiocyanate and the Lumiere from the ammonia. Here is a link on solution-physical developers - http://unblinkingeye.com/Articles/Ph.../physical.html Gets over my head pretty quickly, so maybe someone else will chime in.