If you want a rigid wing, Spruce spars make it so. The LT-40 has Spruce and I think so did my 4*120. One thing I recommend if you are going with a bar as the joiner over a tube, make sure your wing box is well glued, and make double sure the webbing goes to the edge of each spar, it should go to the top and bottom of the wing. In fact I would use 1/16th or better ply for the webbing and maybe even pin it to the spars. I lost my 4*120 ARF due to the webbing not being 100% attached to the spars. They only came to the inside of the spars and after a few flights separated from the spar and eventually cracked the ribs enough that the wing failed.
One other thing, the front pin used to secure the wing to the fuse, I would not use a center pin where the inside rib is part of it, I would use a dowel secured between two ribs with hard balsa. Again, the 4* was like this and eventually it too had cracked after a couple flights. My T-Clips is setup with the dowels between ribs and it is one tight flying wing.