I build rectangular wings wing 1/4" x 1/4" hard balsa spars and leading edge, 3/32" balsa ribs, preformed trailing edge stock and 1/8" thick balsa elevons.
The wing measures 36" long with a chord of 9 or 10 inches including elevons.
The wingtip ribs are 1/4" thick balsa and I use double sided tape and screws to attach winglets / fins so they break away instead of damaging the wing.
The center rib should be thick enough to anchor a length of arrow shaft or FG kite tubing. Make the tubing extend past [over or under] the trailing edge of the wing and glue a dorsal fin there if you need tail weight. This also makes a nice handle to grasp for launching the plane.
The engine / tank mount can be built like a profile plane and glued and FG'd to the wing after the wing is covered...or you can rig your own mount for electric power.
Balance the plane at 15% and use 1/4" of elevon throw.
I'll dig a plane out of storage to show you what they look like.
Oh yah, I like to use Norvel .061s and 6x3 props for power with a Hayes 2 oz tank. This will give you pretty slow speeds and 15 minute rounds.
These planes were covered with craft store cellophane with 3M77 spray adhesive, then the seams get clear packing tape for extra protection from fuel creepage.