You could basically mold a wedge for the flat side to use with a round seat clamp. Get some "JB Weld Steel Stik" epoxy putty. Has a fairly high crush resistance when fully cured and would be easy to form a perfect wedge using the seat clamp in place while it cures. You'll have to cut a slot in the wedge where the seat tube slot is while it's still soft or cut it with a file, dremel, etc. after it cures. It's similar to working with clay so easy to work with. From the looks of your original clamp it doesn't look like it's putting much pressure on the flat side to begin with.
Last edited by Crankycrank; 04-12-21 at 08:25 AM.