Fahrer-Berlin makes good neoprene battery covers:
Unser Akku-Neoprenschutz verlängert die Batterieleistung an kühlen Tagen, bietet der E-Bike Batterie einen Rundumschutz vor Schmutz und Steinschlägen, ist einfach zu montieren und passt sich in seiner Form ideal an die Batterie an.
www.fahrer-berlin.de
In practical terms, I stopped using battery covers. As long as you store your e-bike in heated space, riding an e-bike generates heat inside the battery, making it never really cold. I even rode my Vado and Trance E+ during -6 to -8C frost without battery cover and the battery temperature was well above 0 C! More: I even had a short ride at -14 C without really freezing the battery. (Note: My Specialized e-bikes let me measure the temperature of the battery during the ride).
Of course, there is nothing wrong with insulating the battery but that only makes sense if you were about to keep your e-bike outside for, say, one hour and only during severe frost.