Alan,
I think that's the hold-up with sliders- the tops are somewhat "irregular" in fit. The canvas side-curtains work well because they "flop" a bit. I too was on the quest for good hard side-windows, but I've turned my attention to the top itself, and I think I've just about got it sorted out. The main problem (besides visability), is the area at the back of the side-curtain, where it makes the junction to the top and the top of the door.
Initially it would seem thant the biggest problem is where the side-curtains meet the windshield frame, but in reality, the biggest air leak by far is on the back edge of the side-curtain, aft of the door. Because of how they are made, most tops actually becomes a scoop for air right into the cabin around the back edge of the side-curtain. I'm working on a solution on my car, and I think I've got it figured out- I'll let you know how it goes....