Print both parts and one MiddlePiece of the original shoulder-rig thing.
I suggest to assembly Counterweight and Counterweight_Part2 with first, then take them apart, assemble them again with the actual weight and then attach that assembly to MiddlePiece.
This way it's easier to have the M5 or 1/4" bolts cut their way.
Both pieces are designed such that they clamp the weight securely between 2 ramps.
So the weight cannot shift no matter how much the diameter of it's actual center hole deviates from it's specified 30mm.
All weight rests on the 15mm rods.
Nonetheless you should print these with 0.5 - 0.6 infill to give it enough strength.
You need either 2 M5 wing-bolts for the outer holes or one 1/5" winged bolt for the center holes.