I cannot give you feed back on kayak specific bags, but do know something about various stuff bags in general. Personally, I'm a fan of ziplock baggies. I've used them for many years with the motorcycles, especially if I'm running with soft luggage, which always leak. Individual ziplock baggies keep water problems down. A single stuffed sack that leaks gets everything wet. If, on the other hand, things are also in ziplock baggies in that sack, they stay dry even though the sack is wet. If things get punctured because of roughness or sharps, only a few baggies will leak, not all of them. I like that. It's become much nicer recently as they are now making ziplock baggies all the way up to the 2 gallon size, which is huge for a kayak or motorcycle.

About the stuff sacks, I agree with George about the nylon ones. Price doesn't seem to steer here either. REI has some really expensive ones that don't do all that well because of their design. Cabellas and Sunnies though have some cheap ones that are great. I think Sea Kayak magazine did a comparison article on various stuff sacks a little while ago, and that their article is on line somewhere.

I've got a Millcreek from CLC, and did something different about hatches. You may want to consider this for your CLC. After I cut out the bulkheads, I cut out the centers of them, leaving a 3" ring as a frame. I then made a door to cover it, and used tape to seal it, with thumb screw clamps to lock it down. I'll be the first to tell you I haven't paddled with it yet as the boat isn't quite finished. But I cannot see where I'd have any problems with this. Gives me a great big access hole to my fore and aft voids, which will make working with large stuff sacks simple. Keeps my deck lines clean too.

As for spray foam, I personally just don't really like the stuff. It's too good a water trap. The stuff never fills into corners, leaves voids, is quite willing to crack away from surfaces, especially smoothish hard ones, and does retain water in the first few levels of cells. Water, being the tiny moleculed stuff it is, with its propensity for capilary travel, will gleefully travel along those minute openings, and never leave. I see it all the time on powerboats. Put the hive scraper under the edge of the foam, lift, watch all the water squirt out of the foam, and then it pops up in fist sized chunks, leaving evidence where it was only slightly attached to the hull actually. Boy, look at all the cruddy water trails on the resin surface. Give that water some time, and through the resin it goes, into the wood (be it the nature of the resin or mistakes in application). And now you have a rotted/rotting powerboat. Up at the ends of a kayak, which should stay dry, as opposed to the bilges of a power boat, it may be overkill paranoia on my part. Truthfully, I don't see much water problem with the foam sprayed up under the washboards of powerboats, an area that is not submerged. But get foam good and soaked just once...the voids of a kayak are not well aired.

I think one persons idea of making removable foam blocks would make the most sense if you really want foam. Between shoving a bag up there to blow the foam into, and installing some sort of pulling anchor in the foam as you blow it, you might be able to make a removable foam block. Then you could get it out and let the kayak dry out. If that could be done, then I would think you'd be rather safe, or safer, from having a rotted kayak in a few years. You could also use a skinning paint, like bouy paint, to paint the foam plug. That way it wouldn't leave crumbs, and would look very professional. But bouy paints are expensive.

Something else to actually consider a bit is the stability of the boat if you swamp and are needing some floatation help. No, I'm not saying foam at the ends is bad. But I would say there are better spots. Namely layed along the underside of the deck. If you were to work with some hard foam and create an underdeck layer of perhaps strips (for workability), you would have foam that is essentially out of the way, and in about the best position you could have for stable boyancy of a swamped boat. Powerboats with lots of foam under their deck are notorious for flipping when swamped because of the position of the foam.