Hi,
You can find several waterproof boxes on 3D printing sites. Have you tried any models? Does adjusting the size seem easy ?
https://makerworld.com/fr/models/454290-waterproof-box?from=search#profileId-361858
3D printing a fully waterproof box will be difficult, I use a Pelican case for for my battery, they are very good..
3d printed layers won’t be waterproof.
The advantage with DIY box is that it eliminates the need for attachment straps and saves weight.
Maybe if you laminate it to make it more durable and waterproof
Good luck, a 3D printed waterproof box will be a challenge - 3D FDM prints are porous, it will need many ribs to make it rigid and multiple fastners around the rim, will require lots of work on the sealing surfaces etc, with the air pressure changes with temperature, the box will try to suck in air/water when it cools - Pelican cases have a Goretex valve which lets the box breathe but not let water through…
I actually find that the couple of Pelican Cases I have tried have leaked. I feel that the seal doesn’t have enough pressure on it to seal properly. I liberally smear the seal with silicone grease and that dramatically improves the waterproofing. I much prefer the Explorer Cases (direct Pelican competitor) or even the Camden box.
Google Pelican Case leaking …. It happens a lot !
There is Dichtol Am Hydro for waterproofing