Presuming you don't use a furler, Murrays sells a long bridal fly for a Hobie 17. It is about 15 inches long and has a red plastic vane. this one easily attatches to the front chain plate on the foil and can be bent 90 deg and then 90 degrees back such that it clears the foil post. If you buy the shorter one it wont clear the post and still be able to rotate.

I would advise against drilling into the foil. Performance catamarans has beefed up tangs for the foils that they will give to you (although I am still waiting for mine to arive). I broke 2 foils in about 1 1/2 years when tangs broke.
both foils were properly prebent.

Eric anderson