It sounds like you have not done much research on this yet, but I'm sure DIY plans exist online given the popularity of cold plunges with athletes / bro science people (I don't know that there's actually much peer reviewed evidence on any benefit of this stuff).

The obvious answer would be "make a bunch of ice in the freezer" but I'm sure you could get more complicated. If you do go with a circulator, will you also be treating the water for reuse?

Edit: looking at reddit, it appears a bunch of people just turn their chest freezers into the plunge tank? Check out r/coldplunge