Bite the bullet and buy an Engel, or just buy ice blocks. If you're just weekending, good ol' ice blocks are good for 2-3 days depending on your insulation and usage. If you only put in things that are already cold it will last better. It works for us -- around here we can plan out stops to get to a marina or market that carries ice when we need to. We also have a small l12v fridge that we can use for perishables if the ice runs out.