you can get a regular radiator or hyd oil cooler. and connect into the coolant lines running to/from the radiator on the tractor.
BUT, you need to make sure you put in some way to "purge" air from the extra lines that run into cab. ALSO highly advised placing these purge air valves outside the cab, so ya not smelling antifreeze inside the cab.
you can just get some sort of 12V fan, that plugs into cigarette lighter and place behind the radiator/hyd oil cooler that ya turned into a heater. have seen many folks use a computer case fans,
Newegg.com - Computer Parts, Laptops, Electronics, and More! tends to have a fairly large selection of computer case fans. most of the fans come with min of 2 wires, some times 3 wires, find a computer case fan that has 2 wires to make electrical easier.
folks have hit salvage yards to get various radiators, hyd oil coolers, and/or heater cores. some have poped the dash off of a vehicle, and just took the heater core from a vehicle.
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would forget idea of insulation, and just go for fan blowing heat. and then make sure you can plug up all the "air gaps" were air can move in and out of the cab. removing them air gaps can be a big plus, in keeping heat in the cab. just be carefull of not breathing hyd oil, and like and keep oil cleaned up so ya not breathing it.