Ok....so initial results are promising, though still needs improvement.
I'm surprised at how little the plunger actually moves inside the coil...there isn't actually a lot to work with.
Basically....a locknut is way too thick. That's about...4 washers thick.
I went ahead and superglued (I can easily take it off later with a razor) a normal nut, which is 2 washers thick and tested it.
The nut definitely limits vertical movement and the trapdoor will not hit the wireframe for sure. Also...its just thick enough where it moves vertically sufficiently to catch on the latch. Though...just barely. Its hard to see in this picture, but here is the door just catching on the latch ->
It should look more like this ->
But...it does work to open/close the door.
However...there is no vertical play. Which....you need the door to move upward when its resting on the latch so when the ball hits the trapdoor, the trapdoor moves up..."gives"...so the ball goes in. Technically if the ball has enough force it can go in the trapdoor when its resting on the latch, but...you'll get too many rejects.
So now I'm off to ACE to get a smaller nut, or a few washers that have small enough diameters to fit in the coil sleeve.