You might might to consider a Mamyia 6, not the new version, the old version from the 50s. Folds up very, compact, 75mm fixed 3.5 lens, takes S VI push on filters, lens is very sharp. There are other fixed lens folders as well, but if think your 6X6 SLR is heavy try a Mamyia Universal with a 6X9 back and a 250mm lens.