What you want to do probably won't be possible. It is possible to build a DC-DC convertor for 12-24V DC, I can even explain how to do it.
BUT (and it's a big one) if the current levels are in the range of 60A @ 24V = 1.4kW so if you want to generate that from 12V and you can expect about 70-80% efficiency for the convertor you will need to take about 1.7kW out of your cars 12V which will come to 141 Amps. Your alternator in a landy will put out only about 70A, the rest will have to come from the battery which isn't meant to be discharged for long periods at that rate.
I think car lighter sockets are 1A or 5A so you see the problem. Just to build a convertor of this size will cost a fortune in parts, not to mention the first couple of attempts will probably not survive (but would be entertaining to watch). It would have to be wired into the car on it's own circuit and you battery will not last very long.
Personally I would try and run it at 12V and see if it will still heat, it should only draw 30A ish and you can wire it into your car (might work but will take longer to heat). I would guess it's just a simple resistive element for the heater. Give it a go with a couple of jump leads