I recently converted a foam glider to non powered RC so the comments might help a bit. I chose the Ultra Flyer available at Hobby Lobby. It is made of large beaded foam and is not very strong so needs strengthening to withstand the rigors of being towed or piggy back launched aloft. I accomplished this by adding 2" wide strapping tape (the kind with imbedded strings) spars on the top and bottom of the wings. Other good choices would be to glue carbon fiber strips onto the wings for spars.

Very important is that the spars need to join in the middle and that was accomplished by folding them over the root of the wing where they would be glued back to back when the wings were glued into the fuselage. Popsicle sticks were imbedded into the wing halves for joiner spars.

The tail boom section also received the tape on both sides.

One inch wide strip ailerons were added using aluminum tape to hinge and fasten the ailerons to the wings. The same method can be used for the elevator, but first the horizontal stab will need to be straightened along the aft surface to allow a single elevator.

Elevator servo was placed in the top of the boom and ailerons servos in wing though one servo also in the top fuselage can be made to work.

The balance point with the added strip ailerons is 2.75 - 2.875 inches aft of wing leading edge at the root.

The glider flies well and will stay aloft without problem when finding thermal lift. We've been piggy back launching these gliders with an Ultra Stick 120 carrying them to about 700 ft for release. While I haven't been doing this long, it seems that thermals at our field are generally above 500'.

Weight is 9 oz for a wing loading of around 4 oz/sq ft, which I understand is a reasonable number. I'm not sure powering that glider would have the weight stay within reason.

Note: We've also been folding the wings during launching on those that didn't get adequate wing joiners.

Good luck with your project.