The only issue you'd have with an HDPE container for manufacture is that you could only manufacture cold (or warm). But there'd be no way to heat it unless you immersed it in a double boiler and that would be problematic.
Glass is usually the best bet. Even a mason jar and yes you can big "big-uns". The only issue being, since the bottom is domed with a stipled contact rim around the bottom for what its sitting on (to minimize breakage if you put it on something cold while whats it it is hot enough to cause enough of a temperature gradient to break it, otherwise), heating it is interesting.
I use an industrial hot plate but to keep a 1 quart jar full of water hot (150F), I'm having to set the hot plate to MUCH hotter. Thats how slow the heat transpiration is due to the tiny surface contact area. Think about it. A thin rim that contacts the plate. The surface of that rim is like a bunch of little glass balls. The contact area for each glass ball is tangential, point contact. There's nearly NO surface area to move heat from the plate to the jar.
A pickle jar or similar? They work too but being made out of thinner glass than a mason jar, if you're heating you have to be a bit more careful as to where you put the hot jar so it doesn't break. Simply enough, a nice thick pot-holder on a table or counter is usually sufficient to prevent the jar from breaking as it cools. And here too, they also have that domed, stippled bottom ring thats point contact like mason jars for the same reason. During manufacture, the filled jars are pressure cooked in a huge one (they can get thousands of jars in there for one batch - that kind of big) so necessarily, their wanting to move them out as soon as the cycle is done to start the next, they need to ensure the jars don't break as they cool. Thats under much more controlled conditions than you'd normally have in a kitchen so you just need to be a bit more careful is all. From a hot plate onto a wooden table works, even better onto a nice, thick cloth pot-holder or kitchen towel but folded several times to get it nice and thick for good insulation. I'm sure you'll find something that works well for you.
The one thing this slow heat transipration does do though which is good, is that the inside bottom rim of the jar is hotter a little than the average water temp so you do get a good convection current to keep the cell stirred where I don't find the need for a stirrer (as long as you're not running large currents - I'm running 10ma).
I don't get why you need a pump. Most of us make it in the jar we're going to use to store it or if not, all it requires is another jar, a funnel and letting it cool enough that you don't crack what you're pouring it into (meaning you reduce in the container you manufactured in).
There's nastiness in stainless. Usually chromium (which is VERY toxic), nickel, which is uber toxic stuff,... As an alloy which is what stainless is, those things STAY part of the stainless and don't leach out. If you're running an electrolysis cell and pulling metallic ions off the stainless container, oh yeah they do. Your best bet is "Don't!" (wink)
Its OK to use a stainless rod or wire as a cathode because nothing can be pulled off it into solution but why? A thin piece of bare copper wire is VERY easy to find, usually as scrap you have lying around the home.
Yes, copper in the body over trace is toxic but again too, NOTHING comes off the cathode so its safe.