Is it just a works van thing, going home every night? If so, have you considered freezing some icepacks and just using the insulated (but dead) box you've got now? The peltier type coolboxes never get down to a specific temperature like a fridge anyway, just "cooler than the outside", so icepacks would help out if you got a new one. Passive ones seem to be about £30 cheaper than leccy ones, and will never kill your battery. No help if travelling or without access to a freezer, obviously.
