A single layer of 2 oz cloth should be just fine. I once replaced the ailerons on a 40% airplane using Spider Foam with a single layer of 3.7 cloth and bagged into place. If you are scratch building from you can cut your cores all the way to the TE and while sheeting put a strip of glass between the foam and sheeting. After the wing has been sanded out simply cut the aileron out. It's a great way to get the aileron to perfectly match the rest of the wing.