Hi Tommysta: Wind tunnel tests conducted back in the early 30s showed that an inch of stagger was roughly equal to a third of an inch of gap, in reducing induced drag caused by mutual wing interference. If you add three inches stagger, you can reduce the gap by about one inch, and keep induced drag the same. I like to keep the gap in my bipes about equal to or greater than the average chord, for taper wing bipes. Wind tunnel tests also showed that aerodynamic efficiency of bipe wings was maximized with a degree or so greater incidence on the upper wing, but controllability near stall was not improved, since forcing the upper wing to stall earlier also adds destabilizing pitch-up, as its drag increases much more rapidly than its lift decreases as it approaches stall.

Hi THC: I agree 100% with your quest for lightweight construction, particularly with bipes, which do not carry weight as well as monoplanes, due to their higher induced and parasite drag. I am not terribly big on lightening holes, though, preferring built up structures or thinner sheet fuselage sides with lots of stiffeners. I have found balsa plywood very good for weight reduction, and it withstands crashes extremely well. I make balsa ply by laminating three layers of balsa with epoxy adhesive, with the grain of the middle layer at 90 degrees to the outside layers. It is too easy to use excessive epoxy and end up heavy; I have found that spreading the epoxy very thin, and scraping as much off as possible with a wooden coffee stick produces a very strong laminate, provided that you use a lot of C-clamps and 3/4" ply to squeeze the balsa sheets together very firmly as the epoxy cures.