i have been thinking of making a home made portable heating system for this, having a portable inline heater box.
i dont know how safe it will be but im going to make one.
simply using a aluminium box about 1ft x1ft
having a small gass hob with small gas bottles. with small ezsnap connectors on each side of the box. then have 7 or 8mm copper pipe coiled around the gas hob several times, with a switch to ignite the gas, this should run for about 2 hours on one tank for £2.50 and then just replace the gas tanks. you regulate the flow from the varistream / flowmaster and it plugs inline- have vents in the top in the back of your car or on a small trolley outside the car, should heat the water sufficiently as it flows through the coil, and the copper tube is small enough.
i dont recommend this unless someone knows what their doing but i am going to have a go at it.
would only cost about £20
