In one larger enclosed jon boat that a buddy has, he mounted a small propane infrared heater...12,000 btu's....but I think that would possibly be a bit much for your application.
I carry a 12,000 btu mr heater and a couple of 1lb canisters of propane when I hunt in some more remote area where I have a chance of becoming stranded in very nasty weather. Also a 12x12 poly tarp to create an emergency wind break -tent thingy.
What I use to keep warm on those cold windy days when hunting when a heater is not an option...hand warmer heat pacs....the type you can find at about any sporting goods.....you can even get them to fit down in your boots. http://ecom1.sno-ski.com/heatfactory.html
The kids gave me a pair of electric socks,,,and I sort of chuckled inside when they did....well I extended the battery pack wires up to the top of my waders..and tried them out....heck.. if you run them for 5 minutes here and 10 there the batteries last all day and your feet don't stay cold when it is below zero standing in that blind.
Wear wool also.
Matt