Cheap fish tank substitute, with a bonus:
Grocery store bakeries e.g. Safeway, often sell rectangular cakes that are just about phone-sized, in plastic boxes that consist of black bases with clear rectangular plastic domes over the top.
After you eat the cake, peel the labels off the box, scrub it out with dish soap & water, and you have a perfect enclosure for a phone housing that's slathered in peroxide goop. You'll need another one for the handset & caps, and another for the dial numberplate and misc small parts such as the handgrip cover from the housing.
Turned upside down, these also make decent "project tubs" where you can put all the parts of something you're working on, and stack 'em on your workbench.
The bonus of course, is that you can indulge your taste for bakery goods before converting the box to other uses.
If you want to score a bunch of these all at once and baked goods aren't your thing, you could try asking the person in charge of the bakery department if you could arrange to directly buy a dozen or however-many of those boxes. Or you could go looking online for "baked goods packaging" or some such, and see about getting them from the manufacturer or wholesale outlet.