The recommended spacing works fine using the CLC gasket. It does take some time for the gasket to compress (I don't know, maybe and couple of hours or more depending on temperature). But, you don't want your hatch dropping below the deck and collecting water and I think that's what you'd end up with using a thicker spacer. My CH18 is fine with the recommended spacer. 

Ok, I asked what the thickness should be, and I got a reply that said "the recommended thickness" twice.. Does anyone want to let the secret out and tell me what the recommended thickness IS?

I'm still in the build of my hybrid CH17, but I used one spacer after consulting CLC. My call was prompted by the fact that the kit came with two and the hybrid instructions described using one. I was told that two being included was a "mistake." I am still a bit concerned they sit a bit proud, but I figure if they do, I can cut the rim flush to the hatch to make it a spacer and attach a new rim. A much better option than if the spacer(s) are too wide.

John

Check here: http://www.clcboats.com/shoptips/fitting_out/flush_hatches.html

My hatches are 2 layers of 4mm, so I used two 4mm laminations for the spacers. 

Dave

So it sounds like one layer of spacer (4mm) for the foam seal, and 1 more for the added ply on the hatch cover. 

I'm considering making my covers the way the plans show (except flush) - using 1-ply covers and epoxying radiused pieces of wood to the back. This will also provide a place for the hooks to anchor for the concealed hold-down system.  Any reason not to go this way?