catamaran float

1. Most likely on a canoe, kayaking or paddleboarding forum.
2. Yes you can 3d print them.
3. Yes, use XPS not EPS. Also PU foam.
4. You can fiberglass over foam.
5. No.

Foam filled vs. hollow: hollow floats need thicker skins and can sink if holed. Extruded polystyrene (XPS) and polyurethane foam (PU) can be glassed over for rigidity and protection. Then you can paint them however you like.
See this for building instructions:
Foam and Fiberglass Ama Construction https://amaconstruction.blogspot.com/2021/02/foamand-fiberglass-ama-construction.html
For your size you can skip the plywood spine.

The shape and size are open to your preferences. The compromise is between how much buoyancy you want, how much visibility above the water and towing resistance. Banana shaped floats have low wetted surface but no great directional stability (can be improved with a fin), stick up well above water (improved visibility but worse air resistance). Straight wavepiercing floats have good directional stability, low air drag but a higher wetted surface. So it's up to you what to choose. I would probably try something torpedo shaped, with a resonable amount of rocker so the transoms are out of the water, with towers to elevate the beam mounting points high enough to fit my head under the net. To improve visibility paint the beams and net signal orange and add a big flag.

You can model them on the computer and 3d print, or cnc cut from foam and fiberglass them. Or you can simply attack a block of foam with a knife and sandpaper instead. For beams common Al or carbon tubes, in molded fiberglass sockets, the net tension can hold everything together, or ratchet straps, bolts with wingnuts, etc.