Vat/container material also kinda depends on why you're cleaning them, and what you're cleaning them with.
The only times I've had to clean a weapon that thoroughly was after shooting milsurp corrosive 5.45 in a 5.45 AR upper.
And that only had to be dunked long and well enough to rinse the water soluble salts out of it before normal cleaning.
So I just took it through the shower with me when I got back from the range to run hot water through 'er.
You don't specify harsh chemicals, volatile fumes, or soaking Vs agitated scrubbing.
Just dropping it in for a soak - you could use the PVC pipe above, or you you could knock a trough together whatever size you needed using a 2x6xLength, or 2x8xLength base, a couple 2x4xLlength sides/ends, and line it with a Glad yard&leaf bag.