The best material to make this print is polypropylene, for its very low density (0.9) and for the excellent interlayer adhesion, which make it definitely waterproof. With rectilinear infill at 5% and two perimeters the weight should be around 31 grams. Its volume is around 0.2 liters, the result is a positive boost in fresh water of about 170 grams. There is an even lighter material, the FormFutura Pegasus Ultra Light PP filament, density 0.75, with which you can go down to 23 grams. Because this material is softer, I advice to increase to 3 perimeters the compression area subject by zip ties.