I recently found a small aluminium container in a local Triathlon shop (which I wandered into by mistake, don't worry)
You get a power drink and the pot for £1.85p!
Here is what you buy in the shop:
And here is what it looks like after paint-stripping.
These are very light, slightly narrower and a bit taller than a film pot and, as you can see by the insert picture of the lid it has a heat sealed rubber seal as an integral part of he lid. I have left one completely submerged for 48hrs with ZERO ingress of water or dampness.
That's it.