Problem with flat plate wing is the lack of diehedral.
I added two twin tip fins to make the wind tips.
I put those things on All my RC acrobatic airplanes ; also had them on the
HG that i competed with in comps. They add Lots of stability, but it's at
the expense of a bit of drag, (no Biggie , for the return).
On the glider, there is a Huge difference in performance , if there placed
on top of the wing-tip or placed on the bottom. It looks like you have the
option to go top or bottom (?) ; maybe you could do that on back to back
sessions, and post the difference between the two ? I'd be very interested
in that !!
Thanks : Bille