You don't need to keep your top off water heated. Normally, you're adding very little top off water to your system...it doesn't affect temp. Even more so with an ATO.
You can use a rubbermaid container or even a cheap 5g bucket /w lid as your ATO reservoir if you go that route. Really though, whatever can fit in the space you have will do fine.
I've yet to install an ATO. Currently I just keep RODI
water in a 5g bucket in my laundry sink (where the rodi
system is) and just top off twice a week with the full 5 gallons. I just dump it into my sump. I've got a 125g DT with a 40g sump. Temp never went down as a result.