Yea, I wasn't too sure if the windshield-washer fluid was cool, back in the old days in the bush, we just used whatever worked, and it worked well, and was a hell of alot cheaper than fluids designed for aircraft. As far as heaters, depending on where you are, in Canada we have a Canadian Tire store in every community, and car heaters are readily available, in every shape and size. If we didn't have a hangar, (often) we had insulated covers for the bird and had heaters in the cockpit, one in the transmission bay, and one in the engine bay, and I don't think they use them anymore, but battery heating blankets for our batteries too. Lots of juice required! Our baggage compartments were always full in the winter!
They started nice and were pretty warm when required if you were quick getting started after pulling off the covers, and if electricity wasn't available, we'd start her up every hour for ten minutes, but I'm rambling now. Try any auto-parts store, they should be able to get you something, and while they shouldn't be too close to the plexiglass, there generally not strong enough to do any damage. Once again, good luck.