I had not noticed until you mentioned it again about the profile fuselage. Frankly if you're trying to keep it light I'd dump the profile idea and go for a built up fuselage similar to an old timer or a scaled up rubber model with longerons and diagonal truss uprights. By placing the wood out on the corners of a box like this and usign diagonal truss uprights you can actually build a far lighter AND STIFFER fuselage with less material and at lighter weight. It'll even be stronger since the wood is spaced out further apart so any bending loads will not put as much stress into the longerons as it would a pair of closer spaced sides. I'd use 1/4 sq spruce for the longerons and 1/4 sq balsa for the truss uprights. Leave it rectangular or add a few stringers along the sides and top and bottom so it looks like it has more shape to it. If you're after light this will definetly help. In fact why not go for a semi scale Taylorcraft look with a rather large side by side inspired cross section? IT would look dynamic add to the stiffness by spacing the longerons that much further from each other and generally make the model that much more dynamic. Not to mention all the room inside the cabin for any sort of camer work that you can imagine.

If you're using 2 panels plugged together with a tube or flat joiner then it's actually almost as easy to make the wings with dihedral as not. A couple of small templates and stagger the outer slip fit tube in the ribs to allow for the angle and yer done. Just block in the dihedral before buttering up the slip tube in the ribs and between the spars with a filler mix of epoxy and microballoons or chopped glass. If you use the I beam style alter the first couple of rib bays to a box with caps with carbon or kevlar tow wrapping at the center line ribs and out at the ends of where the blade joiner goes. These are the two critical load spots for a blade joiner. Then make up a joiner from aircraft aluminium (something like 6061-T6. Use a T4 or T6 to ensure the highest yeild point) with the dihedral in the angle and the root ribs angled slightly to match. Simple and no need to even jig it up since the dihedral comes from the joiner and not how a slip tube is included in the wing roots.

Don't pass up on another big spot to save some weight. Using music wire for landing gears is very typical. But the wire size and lengths used on a model of this size can introduce a LOT of weight. Instead I'd suggest using carbon fiber arrow shafting for the leg struts with some form of coupler or joiners at the ends. If the diagonal trusses are lined up to help take the shock loads at the strut attachment points then you can similarly reduce the amount of heavy mounting material at the landing gear attach points by quite a lot. You'd still need some but it can be cut back a bunch since the truss work will pass the shock loads around and into more of the structure.

And finally, for now at least , since you're using two peice wings you may want to consider not worrying so much about the joiner and shoot for the wings attaching via simple mounts on the fuselage sides and use functional wing struts to take the actual flight loads. In such a case the wing panels would mount to the sides of the fuselage that would have an airfoil shaped upper cabin area with a strong compressive box beam that runs side to side with some sort of bracket or light joiner rod to take the rather small lift loads and temporarily hold the wings up while you attach the strut(s). Here again you can make the struts from carbon fiber arrow shafts with a balsa fairing glued to the rear side and nicely made ends bonded in and pinned. Overall this would allow you to not only make the center section joiner structure lighter but to allso make the wing spars a lot lighter as well since the struts are holding up most of the load and the wing portions carrying the cantilevered portions are a lot smaller. Doing it this way even a 144 inch span model could get away with something like 1/2 x 1/8 spruce caps with proper webbing to form a nicely supported I beam. I would then cap the spruce with carbon fiber tapes of about 7 to 10 thou thick for insurance. Light and strong by using the material to it's best advantages.

The use of the D tube style sheeting is still worth using since this is as much for torsional stiffness as it is for covering support.