Issue: I forget ... my "friend" forgets to remove balls before lifting playfield for service and they come crashing down.

I don't know if a product already exists for this issue or I feel like I may have even read about machines that had some sort of mech that would lock the balls in place when lifting the playfield.

I've only forgot to do this once or twice but still, pulling the balls out prior is always a pain. Seems like it would be easy to design some sort of 3D Printed "plug" that you could pop in to keep them from tumbling out.

What clever methods does everyone have to keep balls in place while the playfield is up?

Ah, it must have been JJP that I was thinking of. 99% of my machines have always been newer Sterns so I'm without that elegant solution.