Looking for input and suggestions for the proton pack straps. I have the figures themselves nearly finished but dont yet have a good method for a quick and reliable way to attach their proton packs.
Try sculpting them using aves fix it putty or make them out of the bemdable sculpey clay. 
Meerel and CaptCyan have both made custom straps for matty figures. Someone else also made a custom Janine figure with a removable pack.
CaptCyan made the straps for the Hello Kitty Proton Pack figure, and Mereel made his for his custom figures. (Both have topics) 
removable pack figures are coming from mattel in june. there are pics of em up on the web that might give you ideas how to do the straps.
Can you recommend a brand of bendable sculpting putty.
I still need to smooth a few spots over and fill/paint the spot on their backs. Ultimately I’d like to the set of five to all be gloveless. I’ll probably start working on Peter this weekend but I still need to acquire a GB1 Winston and Egon.
Why not just wait on the Matty sets?
Anyhow, no flexible sculpt will really do what you want it to do. You’re better off just stealing belts from a military-related toy. Big Lots and other stores carry cheap 10" doll-type army toys that have rubber belt straps that should work.