This is an accurate reproduction of the Sterling L2A3 SMG rear cap part, modified for your stormtrooper E11 blaster. The inner diameter is 40 mms and will fit most of the blasters kit in commerce, including those 3D printed.
There are three locking notches inside, in the correct position. The D-ring is printed directly inside the rear ring holder and it's movable.
Any feedback will be appreciated.