I just picked up a Raleigh Redux iE with an integrated downtube battery:
https://www.raleighusa.com/redux-ie-step-over

Having a hard time finding a universal battery cover...where to buy?
Also at what temp should I put a battery cover on?

I've never seen a model-specific cover for a battery. I'd charge and keep the battery in a warm area, and fabricate some kind of blanket to wrap around it (probably using Velcro).

I found some computer cable organizers made of neoprene and velcro that I just ordered.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I ride my winter ebike EPLUS in subzero temperatures everyday in winter for hundreds of kilometers every winter.
I built battery box for this bike from ABS plastic sheets with aluminum plates covers , I dont see any need for battery cover unless it is goosedown winter jacket thick woolen juniper. But it would take so much room