I used to get ice from an ice sculpture company. It's been a while- I think it was Colorado Ice Sculptures
I always called plenty ahead of time, brought my cooler in, and they cut it to the dimensions I wanted. Usually, I'd get a 6-8" slab across the whole bottom. I think a couple of times I got a 2" slab to go on top of food, too, and once had them cut me a wide u-shape to fit in the cooler.
It was crystal clear, and lasted weeks. Did some very hot trips.