I'm not sure if we are talking about the same thing when you say "the 1/2" wide hatch flange/channel".
From memory with the original CLC designs the hatch cutout had no flange,the seal mated direct to the deck.
In the newer plans ,CLC then correctly suggested using a 1/2" wide x 1/8" flange glued to the top of the deck cut out.
This was obviously to help to create a more waterproof seal.
I see it like this, the 3/4" wide sealing strip of rubber not only seals on the 1/2"wide flange,but if you position the foam seal so its flush with the outer edge of the hatch cover, when compressed the over hanging 1/4" also seals on the deck .
There for creating 2 sealing points.
Thats why you need 5/8" to 3/4" in thickness , and to work ,it HAS to be a soft, closed cell foam.
The narrow flange ,in this case 1/2", actually acts like the "O" ring principle,in reverse .
In this case the soft element is the flat rubber seal and the hard element is the 1/2" flange.
On our old yacht ,with hatches which were made out of teak , a 1/8" brass strip was let into the bottom timber of the hatch and left protruding out 1/8" ,and that created the "O" ring principal when it came in contact with the rubber seal on the deck upstand,(I've also seen it reversed.)
I've built 5 kayaks,two with the above described CLC hatches,the other 3 with flush hatches using a reverse "O" seal of my own design.
I have to report that the CLC hatches using 1" wide x 5/8" thick closed foam and 3 external hold down straps ,do not leak.
The others do,I think primarily because the foam is too thin (1/8")and is not thick enough to comform to any slight descrepensies in the deck/flange.
Unfortunatly using a thicker foam now will make the hatches protrude above deck,negating the flush hatch look,but at the same time , probably giving a better chance of a decent seal.
Dont throw away the supplied seals yet,you may need em.

Hope this helps.

Here's to waterproof hatches and dry gear!

Cheers Paul

.

/ I had posted below about needing PROPERLY SIZED foam strip for my hatches.
: Several of you suggested that I need neoprene, and I want self-adhesive
: material that is 5/16" thick x 1/2" wide.

: CLC provided 3/8" thick by 3/4" wide. That's just stoopid! It's
: clearly the wrong size. One might argue that 3/8" would compress over
: time, or with exceedingly heavy hold-down pull. But, the hatch kit makes a
: 1/2" wide flange/channel -- period.

: I don't have a marine supplier nearby, and I have wasted HOURS on the web
: searching for this material. I know it exists, because I've found
: industrial suppliers that only sell in bulk, or, one place with a minimum
: order of $50 + $15 shipping.

: I'm really resenting CLC not providing the right stuff, for both the time and
: money involved in doing it right. This is a heck of a way to conclude an
: otherwise satisfactory build. Can anyone direct me to a known source from
: which I can buy 5/16" thick x 1/2" wide neoprene tape with one
: self-adhesive side?

: Thanks