I've built a FlyBaby and worked on a Piet. Within specification limits for wood density, try to get the lightest materials in spruce and plywood. There is a lot of it in the Piet and care in selection is needed. Covering can be heavy too. Use Dacron and the lightest finish you can find. Don't use thicker gage metals as they are heavy too. I was a heli aerodynamicist for 6 years and landing speed and stall speed are a function of wing loading - pounds per square foot. Excess hp gives you only greater rate of climb and usually costs weight - engine + extra fuel. Lift devices are only good when extra thrust is available to pull the extra drag. The safest weight reduction is around one's waist!