Caliburn - Lower Stock Reinforcement

This is a reinforcment part for https://www.thingiverse.com/thing:2376150.

I printed my Caliburn in PETG, which is more flexible than the PLA the designer intended the blaster to be printed in. The stock_alt2 part would be spread apart by the clear tube at the end when the threaded rods were tightened, enough to reduce catch's reliability. Rather than print an entire new blaster in PLA, I made this part which keeps the threaded rods and stock_alt2 from spreading.