I found this when looking for the same sort of thing. If you've got the time, it wouldn't be too difficult to build a 35 round version for yourself using that design as a base, a sewing machine and some brain work. I'm going to build one for my own drums in my HvZ games because dump bags can be a little awkward when under fire.
That's the breech gate retention piece, it goes at the front end to the left as you look at the blaster. I've circled it in red on this picture:
(PS: Thanks SGNerf, you're awesome)
You can see it wrapped around the vertical sliding breech gate - there should be a screw well in the case for a screw to go in that will hold it down. It's most likely just under the metal rod in that picture. It's not a critical piece but it does guide the breech gate up and down which might prevent some jams and the like. Hope that helps.