Dude I understand the budget limitations, really I do. However, cobbling something together that uses an open flame, burning who-knows-what for fuel, in a sealed cabin is a recipe for disaster. You'll either gas yourself to death or burn the boat down to the waterline.
You're up against some hard limitations:
1. You'll eat up batteries fast, using them to power any kind of heater.
2. You have no means of charging the batteries anyway.
3. You have no money.
4. You have no supplies.
This leaves you with needing some kind of SAFE, flame heat, and needing donations.
Using a stove for heat works poorly because the heat is concentrated at a single point for cooking, and not circulated around the boat.
I recommend you check here:
Someone may have something you can use.
If you can get a small gift of cash, I recommend these:
Origo Heat Pal
Coleman Black Cat Heater
The Black Cat is great because it's flameless and the exhaust is catalyzed so you won't gas yourself. Just make sure you have an adequate flow of air into the cabin so that the oxygen level doesn't get too low. I doubt your cabin seals that tightly anyway.
The best option would be to get to a dock that has 110v ac power and use an oil-filled electric radiator heater. They cost about $40.00.