I wanted an adapter for the Stryfe that allowed me to use some PVC pipe for the bulk of the stock. I wanted to match the lines of the Stryfe body as much as possible (the left side wall would have been way too thin, so I built it up a little). Having two pieces to the collar allows for a redesign of just that side.
I don't plan on removing the stock, let alone during games, so semi-permanently clamping it with the screws was favorable to some spring-loaded design. Once screwed together, it's very solid and it's not going anywhere.
Edit: I added a simple butt end. I found that an 8" piece of PVC was perfect for me, but I'm also pretty tall. My 7yo liked it much smaller at 4". I also built up the left side of the main adapter a bit more, opened up the screw holes a little (no drilling necessary), and chamfered some of the edges.
The pieces should sit flush against each other, though my pictures show a gap due to the raft and brim lifting during my print. I have very conservative speeds and my print took about 3.5 hours for the blaster side, printed simultaneously, and 5 for the shoulder side.
I printed with two perimeters and three top/bottom layers. I would probably bump up perimeters to three or four, in the future.
The holes and recessed areas for the screws were very snug and required a hex wrench to get them through, initially. I drilled them out a little for ease of use.
I found the PVC was very, very snug and it's unlikely that I'll bother to cement the stock adapter cap to it.
(4) M3-0.50x20 mm socket head screws and matching nuts for adapter
(2) M3-0.50x20 mm socket head screws and matching nuts for butt pad
(1) length of 1" PVC for the stock itself (OD 1.315 in / 33.40 mm)