I was looking into these for a short while myself. What I noticed is that some of them get up to +100℉ and that will cook your brew. I don't know about that model specifically, but the Q&A on that ad says it's 95-100.
A crock pot filled with water around the jar would be a decent solution, but you'd need to use a controller to turn the pot on and off, or it could boil your brew.
I use a 10-gal aquarium that I upgraded from, filled about halfway up with water, and a cheapo pre-set submersible aquarium heater keeps my solutions right at ~78℉. It turns itself on and off. Seems tropical fish like a similar temp range as kombucha!
If you're not wanting to spend any money, and don't have any other options, a box and night light may be your best bet.
Let us know what you come up with!