Make good wood joints. Glue can not be used to strengthen poor fits. If you have a piece that doesn't fit correctly, cut a new piece that fits then glue sparingly. Glue is heavy. If you want to build a model to minimum weight, make good fits between parts, build with as little glue as possible, and use light, stiff wood.
Champions match the rubber motor to the propeller.
Experiment with different widths and loop lengths of rubber. Keep good records of your flights. The most duration will come with a flight which climbs slowly from the floor to the ceiling where the model will scrub the ceiling or bang around in the girders for a while, then drop below the ceiling and cruise at the same altitude for a while and then slowly descend to a landing with about a single row of knots in the rubber motor. If your model beats itself to death in the rafters, the rubber motor is too wide or the length of the loop needs to be increased. If the model does not get up to the ceiling, just reverse the process.