Advice on Plastic Casing/Enclosure

Hi,

First time poster. I'm trying to find a casing for a project I am working on. Ideally if it works out well, then I will get thousands more made, so it's not just a 1 off.

I just need a plastic box of certain dimensions that is waterproof (in saltwater), has a lid and has a handle on one side, and needs to be relatively durable (nothing too heavy though).

I don't really know much about plastics at all, so am just seeking advice on what my different options are. So far I've investigated a few:

1. I've found a couple of electrical casings that are very similar size and will do, but I'd like a few variations. Is it easy to work with these? The size is fine, I just need to put on a handle and a clip (it needs to slide into something and then clip in), and then machine off the little ribs for the circuit board.

2. I could go to an acrylic worker and give them the dimensions.

3. I've asked an injection moulding place and they said a mould is usually around $20-$30k AUD which is not possible.

4. Looked into 3-D printing

I'm just hoping for pros cons on each, and any other suggestons I haven't thought of before I go down one path in too much detail. Open to any other options that people might have, as I have no idea on plastics