Help me keep my fish fresh.

2-3 times a week I go out to my favorite surf fishing spot. This spot requires a 4 mile walk on the beach. I pack up a modified old climbing pack, walk down there, and spend a few hours before coming back. The best among us occasionally kill a nice fish trying to retrieve it and I can't stand wasting fish. I'm experimenting with different strategies for bringing them back fresh. I use a nifty floating ring I made with a big mesh bag and a pool noodle for my livewell so I can keep the fish till I decide which ones to keep. That works AWESOME. On Sunday I caught a trophy spanish mackerel (yes it was from the beach) and killed the thing getting the hook out. Incidentally, I'm done with treble hooks. They are just WAY overkill. Basically I want a way to freeze fish in that situation and for the walk back so I don't waste any good fish. I just want the lightest, most compact solution for freezing the fish out there. My immediate thoughts were reusable chemical cold packs and some home made soft cooler. I could even setup the cooler so the insulation foam could double as a beach seat. BUT I, obviously, want the lightest, most simple, and compact solution possible. I usually bring back between 40lbs and 100lbs of redfish, specks, white trout, pompano, sheepshead, spanish mackerel, flounder, etc. and 2 or 3 cold packs just won't cut it.