[dcb]

If you want to save some heat for later you could do like the prius.

If you could find a couple thermoses at the thrift store, it might be worth it to try to capture some hot coolant when you shut down and reintroduce it at startup. Needs plumbing/pumping skillz, but could be done pretty cheap. Might want to limit how many psi you expose the thermos(es) too, and don't make it leaky.

Could also use a cooler or something, might cheaper and larger capacity, but will be less insulated and bulkier.