I have a quick fix that works great, but it can be a little inconvenient. I got a SealLine seat, which is basically a fancy Thermarest pad (similar to what Pygmy uses.) I let it fill itself up, sit down, and slowly let air out until my butt just begins to touch down. That takes care of my posterior.

However, by itself it wasn't enough. My legs got no support, and I was in agony after 30 minutes. Then a friend taught me a trick. I put a little air into my paddle float, and when I sit down I put it under my legs, butted right back against the front of the seat. If you get it in the right place with the right amount of air, it's great! It gives your legs just the right amount of support. I paddled an 8-hour crossing to Catalina Island, and my butt and legs didn't give me any trouble at all.

It's a hassle because sometimes when you get into the boat and slip your legs forward, you push the paddle float up to the front. Then you have to start over. Also, you've got to get it adjusted properly before you launch, because it's virtually impossible to adjust once you're afloat.

I'm going to try something different on the Guillemot I'm building for my wife; maybe the sandbox-mold-fiberglass seat that others have described. Maybe I'll try carving a big hunk of minicell foam with a disc sander. (She doesn't yet know she's going to be my guinea pig.)

I just thought you might be interested. I think you'd have an easier time of getting the paddle float in the right place and adjusting it underway in your Mill Creeks than I do in my Chesapeake, since the cockpits aren't so confining.

Good luck.