Since Optima YT's have problems with cold weather, I decided to install a battery heater system in my Sparrow. I didn't want a super complex or expensive system, and I had to fit it into the existing battery boxes - which have very little extra space.
The solution that I came up with was to use some heat cable under the batteries. This cable self-regulates it's temperature, so you don't need thermostats, and it can't get too hot. To keep the battery weight from damaging the cable, I placed a 1/4" thick sheet of plywood in the bottom of the battery box, and used a router to cut a 3/16" deep by 3/8" wide slot in the wood.
In my tests so far, it seems to keep the batteries in the mid-60's on cold days.
The pictures below show the heater boards in place in the front battery compartment. To power them, I cut an extension cord in half and wired one to each heater. This lets me daisy chain the heaters from a single outlet. In the Sparrow, there is another heater board in the battery box under the seat, and a small one under the single upper battery in front.