3d extrusion prints have microscopic holes inside, it’s porous. I wouldn’t trust it depending on the application. You can seal the outside and they also sell additives to lessen the effect (here’s a video about it: https://m.youtube.com/watch?v=Q8x-mjjT8j4) but casting resin might be better (haven’t tried). Resin printing doesn’t have these problems from what I understand but ou can’t really stop the print midway.
Fiberglass, if the object isnt complicated to make, would be the best option in terms of quality. Bending and cutting pvc sheets into a box then soldering is also an option. Soldering pvc and rolling fiberglass isn’t really difficult but they both require specialized tools/materials. Getting pvc pipes and soldering end caps would probably be the easiest.
How robust you need to make it largely depends on pressure and depth.
“But you said you would be my friend if I did what you said!”