Some tubs come in different dimensions but they're the same quart amount, the 34qts I use for my adults are wider than the longer 41qts I've seen in stores and they are deeper and not as shallow. I may be able to get actual dimensions for you later.

In the meantime while you're saving for a thermostat you can look into getting two temporary dimmers for your heat sources too: they're roughly $10 each at Home Depot (lamp dimmers that you can adjust accordingly). Not as good as a thermostat, but they'll do the job in the meantime. I don't have experience with the ultratherms you've mentioned.