I think most guys just drill the holes oversize, fill them with thickened epoxy, then re-drill the holes. You can put them in, let them cure and if necessary sand before you glass the blank.
There are also i think nylon inserts avalable from
http://www.dovetechaquasport.com/
They are for standard wakeboard fins. If your fin uses two screws you'd want something else.
Trent