No oven? Just turn on the light. No water heater? Put the product on top wrapped in a towel.
That is not a particularly narrow range. The real challenge is that you are at the top of the food safety danger zone so most products will be designed to be hotter than 140°F. You could maintain that with a laboratory heater and certainly with a manual (off-low-high) slow cooker and a dimmer like this https://www.amazon.com/Lutron-Credenza-Halogen-Incandescent-TT-300H-WH/dp/B0000BYEF6/ref=asc_df_B0000BYEF6/?tag=hyprod-20&linkCode=df0&hvadid=167140365824&hvpos=&hvnetw=g&hvrand=4853017820206176086&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007937&hvtargid=pla-274342005344&psc=1 . You’ll need a thermometer to get in your desired range but it should be stable and repeatable.
Do circulators commonly get down to 130F?
For clarity, @shrinkrap is quoting from the CI article and should not be held to account for the words. I think they are both accurate and funny. Instant Pot is a cult. Step away from the Jim Jones Kool-Aid™.
Also from the article:
Unfortunately the CI analysis did not include graphs for the standard conventional alternatives.
