My latest seat mounts have been welded on. Don't know if you have access to a welder or not.
Made the seat pan out of 16 gauge steel. Welded the rear mounts directly to the frame.
Latest one I did was make a new front mount/battery hold down.
I welded rear mounts on again.
This is what my seat used to look like. Bolted brackets to the crossbar that would reach the studs on the back of the seat.
You can use this set up to mount the front of the solo seat. just screw the mount to the top of the battery hold down.