Nothing that a peice of tupperware, some fittings and some silicone can't do (really cheap too). Just remember to ask Mom/GF/Wife before raiding the kitchen cupboards (Or Dad/BF/Husband as the case may be).
I remember stealing Mom's clothes iron to wax my skis once without asking her. Ohhh Boy there was trouble that day.
Hey! I found, I found
An "ADDiS Food Saver flavourtight seal" plastic box. It's 16cm in length, 11cm width and 7.5cm height.
I filled it with water and turned it in various angles. Upside down it was completely tight, also sideways (with some miniscule leakage from the corners), when I turned it so a corner was at the lowest point, there was some slight leakage.
I'm worried that it might be too small though, so that the pump will heat up the water too much (This is only 1.3 liters, and I was tinnking more like 2-4 liters), or has this little or nothing to say?
I also thought metal tanks were better to avoid excess heat buildup inside.