Just plug a heating pad, crock pot, or something along those lines into something like this. Should be a lot more accurate, and repurposable for other uses.

Inkbird Itc-308 Digital Temperature Controller Outlet Thermostat, 2-stage, 1100w, w/ Sensor https://www.amazon.com/dp/B011296704/ref=cm_sw_r_cp_apa_8VqdybM4N0JJV

Something slightly different for the lazy: http://www.amazon.com/gp/product/B011296704?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00

This is the one that I use (Inkbird ITC-308):

https://www.amazon.com/gp/product/B011296704/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

I have it and an anova. I like the inkbird for multi-day cooks because I don't like hearing the anova's motor running for days. Also, having two sous vide options is good for planning if I'm going to sous vide vegtables and meat for the same meal.

I cooked a chuck roast at 131 for 3 days with the inkbird in a crockpot. It worked very well. You want the crockpot on hot setting with water until it gets close to the target temperature, then turn it to the "keep warm" setting. On keep warm, the crockpot + inkbird kept it consistently at 130-131 the entire time.