Well, I could do a lot of things. I could make a heavily insulated keggle MLT. I could add a ton of foam to a SS pot. I could just keep using the cooler MLT I have, even though a large ridge forms every time I add hot water.
Right now, I'm looking for a relatively inexpensive SS pot to use as a liner for my cooler.
Have you tried a thread/google search? If a pot that works as a liner existed, I would think there would be threads/info about it, and maybe even LHBS selling them. If searches don't turn up anything, a good fitting pot may not exist, or is hidden so well it hasn't been found yet.
It would help if you would state what the intended use is. I just assumed you were trying to line your water cooler to create a SS insulated MLT. In that case, I would think that finding a fairly close fitting liner to maintain volume/capacity is a priority, in which case it may be a difficult quest. If the goal is to keep a pet turtle warm, maybe max volume isn't as important. Maybe you just really love the aesthetics of ribbed orange cylinders.
I only suggested finding a pot, and then insulating it, as an alternative solution that doesn't involve searching the ends of the earth for a pot that fits precisely into a cooler. Sometimes people become fixated on a particular solution, when there are other easier, less time intensive, though possibly more expensive solutions. It is hard to know without knowing what the problem/goal is.
Is your goal to have a 10g (exactly 10?) insulated SS MLT, but with the design constraint that it non-destructively use a pre-existing 10g water cooler as insulation for economic reasons? Also, if you value saving money much more than saving time, it would help to state that as well. Or maybe you value time and money, which is why you are on here having others spend their time doing the searching, and not yours. Have any fences that need white washing?
I don't know if you have already checked the usual places- Home depot/Cabela's/Bass Pro turkey fryer pots, bayou classic pots, etc.