For the GH3, Blanches viewfinder solution is okay, but imho not for the GH2, because it's display is not good enough to find focus. You'd just collect more useless gadgets (like your toy mattebox, which even doesn't seem to take filters). Yesterday I read a review in a german cinematographers magazine of a bunch of the 'best' DSLR rigs (Chrosziel DV Studio, 2200 €, Vocas DSLR pro, 3500 €, Redrock eyespy deluxe, 1400 €, Kinomatik Movietube, 4000 €), and the verdict was, they all had to be rebuilt to work ergonomically with the particular DSLR and for the camera operator. Only the Movietube had the counterweight right. Even the pro admitted that he would rather buy parts and build an own rig according to his needs.
I promised to post a DIY big comfortable eyepiece, and I will. I've been testing materials and improving my prototypes, and in a few days I will present a photo tutorial with detailed instructions.