RE: Cyclon / Fora / Profi .049 Speed Trainer
The inside of the balsa compartment is flooded with thin epoxy [like West Systems] or you can use thicker epoxy that has a couple drops of alcohol in it. Too much alky curdles the epoxy, so just a couple drops and use epoxy that has been warmed up. The epoxy can raise the grain and cause stipples that you can't do much about, but you can "carpet" the interior of the fuel cell with velcro, or thin slices of soft foam padding.
Wings / deltas fly by the same rules as any other plane. The deltas are naturally fast rollers, so go easy on your control throws for banking. The CG is usually set at 15% MAC instead of 25% for conventional planes. The flyable CG range is narrower, but light wing loadings widen that band just like any other plane.
A little trick to seal the iron on covering to the fuel cell opening is to mix a small batch of 5 minute epoxy and throw in a tiny speck of lightweight spackle. It acts like a thickener and causes the epoxy to form a nice smooth, rounded lip at the edge of the opening.