Another economical material is cardboard. Often free but even if you have to buy it, it's hardly expensive. OK, a problem with card could be it's tendency to absorb water and turn into a mushy state. But, This is only a problem if you fail to seal exposed card surfaces.
Brings to mind that back when Yahoo had its Yahoo Groups there was a surprisingly active one for cardboard boats.
Before that time and technology ...
My first rubberband powered submarine was built of cardboard/cardstock from candy bar displays when I worked at F. W. Woolworth in early 1990s.
It was of horseshoe shape cross section, about 30 to 36 inches long; had hull skin on each side of the frames except under its deck; hull interior space was free flooding. The double skinned hull had a thick enough air space that it took 3lbs of Daisy BBs to ballast it.
What the prop was is not remembered.
What the planes were made from is not remembered but I do remember being careful that their corners didn't damage vinyl line of friends' above ground pool.
Cardboard was heavily soaked with clear gloss brushing lacquer. It needed a bit more sealing than that for best longevity but is served its purpose for a couple seasons.