It's easy, and don't sweat trimming back the core a little bit. Most cores are cut a little long anyway, and you get the chord back with the aileron anyway.

I use 5/16" square balsa for the TE. I trim the foam wing back so that the TE is 5/16" thick, then epoxy on the balsa. I then sand or plane down the balsa so that it continues the upper and lower surface of the wing. For most airfoils, the finished TE will be about 1/4" thick, which works well for the 1/4" ailerons I use. For scale planes, I usually use a 1" wide aileron, for open classes, I usually go for a 1-1/4" aileron (or is it 1-1/2"? I honestly forget, and have to re-measure every time I go to order new stock).