The prior owner of my boat found rectangular buckets that fit 6 pound ice blocks nicely, and they came with the boat. The melt water stays in the buckets and only condensation goes to the floor of the icebox.

I can even fit 2 6-pound blocks in two buckets, if the ice goes above the bucket tops. I use a bar sink stainless grate above them, so the things that need to be very cold go along the sides of the buckets down low, and fruits/veggies/greens go on top.