I used 12mm copper tube about 8" long and cut lots of slots diagonal across it, cut out 4 discs with 12 mm holes in to slip over the copper tube ( 1 each end and 2 as baffles) that were the same diameter as the 35 mm outer tube and fitted all that wrapped in steel wool in the tube about 13" long, made a reducer to fit over the barrel and bingo, working sciencer !
Not filled the stock yet, I'm looking to fit the Mancraft reg and co2 reservoir in a kinda box that I'll make that fits in the stock, but I'm still waiting for it to arrive then I'll add lead to get the balance perfect then finally fol any voids with squirty foam,