Check out kwong2001's thread (Check out my dad's SPAD) on the SPAD Aircraft-Coroplast design site. Simple bolt on plates are shown in the detail pics. You can easily make tip plates and tips matching your wing airfoil to experiment on the effects with and without tip plates.

I believe effects of vortices would vary depending on the wing tip shape. For a large tip on a low aspect ratio/constant chord wing (i.e. wide tip), the vortices would be exacerbated over that of a tapered wing with a small/narrow tip. Tip plates would then be more advantageous for the larger tip over the smaller tip. Just my guess though.

On the other hand, tip plates would aid in stall resistance which in turn may inhibit aerobatic maneuvers which require the wing to stall (spins and snaps). Then again, thats just my guess.