These will probably be 1/4 inch bolts, so drill with a 1/4 inch drill bit.
If using 1/4-inch bolts, the tap drill is appreciably smaller, about .195. A 1/4-inch (.250) drill won't leave much material for threads.
Rubber bands are just fine for wing hold-downs. Let's not forget early sport and even pattern planes used rubber bands on the wings, and sometimes even landing gear, Not a bad idea to put a couple extra on each side in case of a failure.