I have a genuine Magpul GripCore for spare bolt and firing pin. It had problems with staying in the grip (ie. fell out during shooting or during transportation of the rifle). I tried adjusting the tension on the locking tab and it broke. Rather than buying another one, I decided to design my own.

I also improved upon the original by lengthening the bolt carrier portion to allow carrying a spare cotter key and added a hole to carry and lock in the bolt with the cam pin.

Update 05/04/2024: increased strength of the latch and fixed issue of supports not printing with 0.4mm nozzle. Made a few minor changes to the print file for better overhang results.