I will venture into the same project as you this winter, and I think the easiest is plywood.
Alu tubes are a pig to bend, and I am unsure how to bend plastic (water) pipes without collapsing or weaking them, and if they are strong enough if glued together.
My plan is to cut two strips of nice marine ply into a nice back formed shape, about 2 inches wide, then glue and screw thinner, more bendable ply against the edge that will rest against the body.
Two, one left and one right, then connect those with three similar t-strips, one in tip, then middle and one on bottom.
I do not know if the idea is stupid/crazy/unworkable though.