Hi
I have been in the electric heating business over 30 years and have a few concerns.
First, simply wiring up an element of some sort may or may not heat the space depending on enclosure design.it simply depends on heat loss of the enclosure as compared to the wattage of the element.
A homemade design such as this could certainly create insurance issues if something does go wrong, what you are making is not UL ( or CSA ) approved and if it is determined to be the cause of a fire, it allows your insurance company to simply walk away from any liability. All space heaters have thermal high limit cutouts incorporated into their design, the idea is that the electric power to the element is disconnected in event of an abnormal over temperature situation so at least there is some kind of safety built in. Unfortunately a space heater may have clearance requirements that may not suit your enclosure design, again not adhering to label warnings or installation instructions may get you into trouble.
I would suggest perhaps an alternate method, perhaps an approved heating belt around each carboy?