I gotchu fam.
Part 1: regular HS shaft collar
This one can be improved by tightening the HS shaft collar tightly, so the collar can hug the motor a bit tighter. I didn’t tighten mine all the way, so that’s why it slipped off. Thanks @sazrocks for the tip.
Part 2: low profile HS shaft collar
Part 3: modifying old lock bar.
Cut the sides off, and attach to motor
It doesn’t slip under load ( I was running it at full power while blocking its motion)
This can definitely be improved by cutting the lock bar so that the mounting holes are partially there, allowing for a gear mount of some sort.
Part 4: 4 post hex retainer
Hollow out a 4 post hex retainer with a 1/4 bit, and shove it on. It sticks relatively well, and as a bonus you can put some c channel on there.
However, machining lexan pieces might be the best bet for these.