If the issue is that the clamp just doesn't clamp tight enough, but there is space - the slit in the collar has space to tighten - you could try tightening it up more? (Yea, I know you probably tried that already.) If the collar slit is drawing together so cannot tighten further, have you tried sanding that down to make some space to allow it to tighten more? (I can't imagine getting a bike with a clamp that closes before the post is tight but anything can happen, I guess.)

I mean, a clamp is a clamp is a clamp. As long as there is room for the clamp to draw together more, you should be able to tighten it more. You could also try unscrewing the qr bolt and handle part of the clamp and replacing that with a bolt, a few washers and nut, converting it into a bolt on.

When I had this issue on my CF MTB, I was afraid to clamp it tighter as it is CF, but I gave it half a turn more, and used that paste that is for stopping CF from slipping (looks like grease but has tiny flecks of grit in it to provide friction) on the post, and the two together stopped the slipping.