Another option for the middle: add some velcro on the underside of the seat and to the top of the fiberglass seat box. You would likely have to use some mechanical fasteners to keep the stuff from pulling up but that might give enough holding force to keep it down when underway. Use staples on the underside of the seat and a couple screws through the velcro on the seat box.

Buddy has the same islander. The problem you cant "really" fix. It is due to the wind channeling threw the opening under the whindshield. Wait till your carpet comes flying up ;) You can try and replace the velcro that is on where the seats rest and add it to the bottom of the seat that helps hold the side seat(s) in place.

I made up two straps with some nylon webbing and put snaps on the end, I fastened the strap to the seat and drilled in some snaps in the seat frame box and it seems to work perfectly.