I like that window edge idea, have to keep it in mind, if I'm ever without a stone. But if your truck is there, couldn't you just pull out the dip stick, and get a few drops. Should work in a pinch, as long as you have your stone. If you have your stone, but the truck is 5 miles away and you just have to get that bull moose quartered before dark, plain old water will get you by.
