Ford's meat locker in Provo layer froze some coolers for us for us about 5 years ago for a July Grand trip. It was a novel request for them, but they were happy to take our money. They have a huge walk-in freezer--like an entire basement. We filled the coolers with our food and had them add a couple inches of water a day, and picked them up about 6 days later.
Duck taped them shut and wrapped the coolers in burlap that we (or the rapids) kept wet for an evaporative air conditioning effect. Clearly marked with "DO NOT OPEN UNTIL DAY X"
Opened one cooler on Day 12 of a this July trip, with plenty of ice still intact. Once you open them, it disappears fast though. And this was just a regular Sam's club Igloo cooler, nothing too fancy. I wouldn't use this technique for days 1-6 or so, though or you'll just have to leave your cooler open for half a day so the block will unfreeze. Just use jugs or blocks for the coolers for those days.
If you have a meat locker, cold storage or walk-in freezer connection in your neighborhood, strongly recommend this approach for a Grand Canyon Trip.
Its honestly a bit overkill for a Main Salmon trip though ... just freeze jugs of water in a chest freezer and they've always lasted until day 7 or 8 for us if you keep the coolers shut -- and since its jugged, you can include that in your water supply.