Way back in 1976, I decided to finance my hobby by building a large amount of r/c sailplanes and selling them. I chose the Mark's Models Wanderer (Mark's Models is now known as Dynaflite) as I could get them as cheap as $14.00 and I'd already built a couple of 'em for friends. So, I bought 20 kits and began to build them all at once in my garage 'shop' (actually just a dusty, little room off the garage - cough, hack!).
I tried a number of 'modifications' on the wing, fuse, and tail to improve performance. What I found was that the following modifications were most helpful in improving the flyability of the Wanderer and could probably be applied to other models as well. These modifications are so helpful that they make the Wanderer into quite a nice, floater-type, slope plane. You can get nice pseudo-axial rolls and some inverted flight characteristics out of this plane. Having a two-piece wing is really helpful too, especially with a two-seater car.
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Stringers
I added what I called 'stringers' to the area of leading edge between ribs. These stringers help improve the consistency (I think) of the airfoil on the wing, especially because it eliminates a lot of the 'covering sag' that occurs between ribs. I've used different sizes and wood types, but what works probably best is to use strips of balsa 1/8" square. I used to slot the ribs before gluing them into the wing by stacking up all the ribs, pinning them together and cutting 2 slots on the upper surface of the rib (just enough to hold the stringer) all the way down the stack of ribs. It helps to do all the ribs at the same time (i.e. both wings). Where should the slots go? I dunno, I never measured. I've built some with only 1 strip, but 2 seems to be better. If you put in 2, put them equally spaced between the leading edge of the wing, and the thickest part of the airfoil. The figure below illustrates this (over 2 ribs, you'll want to do the whole wing, except [maybe] the wing tips):
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Semi-Full Flying Rudder
I improved the rudder effectiveness by 'slotting' the upper section of the non-movable part (the fin) of the rudder and attached it to the moving part. This change allows the Wanderer to roll and do some of the most amazing things on the slope. So much so that you'd think that there were ailerons on the wing. The figure below illustrates this:
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Wing Rods, or a 2 piece wing
I built the wing in 2 pieces with 2 steel (piano) wire rods and brass tubes. This allows the wing to be broken down into 2 three foot wing panels so that you can actually get it to fit in the trunk of your car. These days, I'd probably use carbon fiber wing rods as they're about an ounce lighter, but only if you can afford $3-4 per foot. I used to make one rod bigger than the other as, really, only one rod of about 3/16" in diameter (if you're using steel) is necessary.
