They sell a battery warming blanket up here at Canadian Tire. I think it's about 80 watts. Like a glorified heating pad. Heck, you could probably use a regular heating pad but it may not last. It wraps around the battery but you need somewhere to plug it in.
Google battery blanket. You can probably get them at most auto parts stores.
Here is someone using it rather poorly on a traction battery:
Battery Blanket - GreenHybrid - Hybrid Cars
Really, in that application, it should be underneath the battery or covered by non flammable insulation.
EV guys also put their batteries in a box insulated with hard foam. That plus a heating pad and you will have a very happy battery.
I'm really noticing the lower capacity now too. I have been looking at a warmer too but now have the Prius to get working so it's on the back burner.