Nowadays most loosefooted mainsail clews are held down with a velcro strap around the boom - simple. Simply a back to back velcro strap that goes around the boom and through the clew a couple of times.
The only issue might be anything on the boom that might prevent the strap from sliding as you adust the outhaul.
The image is for a Laser, but the idea's the same. Any sailmaker can make one up for you for $10-20 I'd imagine.