I love the double deer eating popcorn. But my all time favorite is the Panda.

Bob's spar will work fine in this model.

For the pull apart feature of the wing, no need for "I" beam subspars. Let me suggest carbon tubes. The carbon tubes I use are 7/8" OD x 1/32" wall x 30" long, for a 2 meter wing (78"). Very light and strong, even for a 6HP powerplant and 16 lb weight.

For this project, a pair of 5/8"-3/4" OD tubes x 1/32" wall x 24" long is plenty of strength but assumes the pulling apart happens in the middle of the 108" span. The tube sockets will need to be built inside Bob's main spar and inside the shorter subspar at the 9" station of the chord.

Fifteen pounds is achievable but I'd hazard a guess that the OP may not have the experience to build light and strong. Some of the suggestions we are making should help in the light and strong area (I've been at this for 45 years myself just like Bob). But even at 18 lbs, if the wing area is around 1500 squares, this plane will fly superbly.

To the OP, let me suggest that you add some more carbon tubing longerons on the fuse, all around. 1/4" x .020" wall will give great strength and should help reduce the balsa longeron breaks when you handle the model. With added longerons, 1/16" sheeting is all you need. And if you plan to tissue and nitrate dope the covering and paint for finish, even better....