Why does it have to be acrylic?
Boxes of such a small size might be available from craft supply places, but they would likely be made of polystyrene. There are also boxes sold to display items in stores.
I would guess it is unlikely you will find something of the exact dimensions you want, especially if it has to be made of arylic. Most small boxes I have seem are polystyrene.
Small aquariums in that size range would probably be made of polystyrene also. Large plastic aquariums would probably be glued up from acrylic sheet (AKA Plexiglas).
There are also thermoplastics like polypropylene and polyethylene. They are opaque, not clear and can be welded, but not easily glued.
I have made small boxes, similar in size to what you describe, from acrylic for taking pictures of small fish (≤ 1 inch).
Gluing acrylic is not difficult. There are two main gluing techniques:
- Use a running solvent for solvent gluing; this requires really well cut edges (often using a router, not a saw (which leaves a lot of grooves in the surface from individual saw teeth), or sanding smooth after cutting) to get a very smooth edge, and therefore a good fit. Apply a few drops of solvent to the joint after assembling the peices and let it wick into the crack.
- Using a solvent glue with a bunch of acrylic dissolved in it (like a paste). This is sloppier looking but fills the gaps very well and is less likely to leak.
If impact or heat resistance is important, use polycarbonate (AKA Lexan). Acrylic is comparatively brittle.
A plastic shop like
@berkeman suggested could probably supply you with any plastic you need.
Plastic shops often have scraps of plastic (leftovers from jobs they have done) you can get for cheap.
