How about getting an older Formula 18 main and cutting the bottom off of it. there is not too much foot difference if you needed to cut a couple of feet off as the sail is pretty square that low. Buying the sail would not be too much money but the battens would run some bucks. I have sold used mains for anywhere from 300 up, but the 7 battens cost almost that much. Maybe you have some battens that will work from the old sail? Then you are good to go.

If you do not care too much about looks just keep the boom at 11 feet so the sheeting can stay the same and slip a loop around it for the new sail to attach about 8 feet from mast.

Later,
Dan