Folded over card stock is easily strong enough to test as you mentioned. Just fold so it's an open V shape and masking tape it for the trial. It should extend up onto the existing surfaces by a good 1/4 to 3/8 inch and then tape in place. The oil will make a mess of it after a few flights but by that time you will know if it helps or not.

The CG location is a biggie though. Try it.

Also to greatly enhance the rolling speed on these trainers get used to adding in some rudder along with the aileron. That REALLY snaps 'er up!