I have no idea if and how you would be able to make those darn screws stay in place. Is the bottom of the cart (where the screws fit), like skewed or something? Maybe you can correct the angle at the bottom with some kind of tool?

Sorry if my reply doesn't help you much, but the arrangement I use can be tightened pretty damn hard without the screws slipping out of place. Perhaps it has to do with the distance of the holes in the shell?

Maybe this is a long shot, but you would perhaps be able to purchase two of those tiny aluminum distance flanges used for less tall PU's.

If you some how cut one into two halves, and superglue them onto the sides of the PU, in order to hold them %

Glad you fixed it. But if you really needed to add "something" between the cart and the headshell, you might get further improvement of the soundstage by adding a slice of rubber/foam/cork instead. You may or may not believe it, but the improvement is more than noticeable.

My post on PU tweaks