[Gundam-Dude]

a cheap and lightweight method I can think of is tracing the outline of the bow in 1:1 scale on a giant piece of cardboard then cutting it out. buy some insulation spray foam and spray the fuck out of the cardboard and let it harden. then slowly carving out the shape you need with a knife (because it's going to become a giant lumpy mess after the insulation spray foam). then possibly coating the final piece with encapso k to get your desired finishing, but that's probably the most expensive product out of everything.

the cannon on the other hand, I can't think of a straightforward way of forming it since it's cylindrical in shape.

not a pro cosplayer in the slightest, but my gf makes cosplay for a hobby, and she uses insulation spray foam a lot for intricately shaped pieces that also need to be lightweight.