I ripped some minicell foam on my bandsaw to the thickness that I wanted and glued it on with 3M weatherstrip adhesive. On my Shearwater 17 I glued it to the hatch cover. On my wife's SW16 I glued it to the hatch rim. On her boat I left out the thin lip that is supposed to be glued to the hatch rim. This was suggested to me by Eric Schade. Both hatches are nice and flush now. I am using bungee hold downs for both. Hopefully they won't leak (much).

Dan 

I used hatch cover tape from Noah's, it is a closed cell foam that works well.  it is very plyable so when you lay it down be careful not to stretch it.  My only complaint is that it tends to seal too well, if you leave your hatch closed for couple weeks it sort of sticks closed.  I have taken to the practice of leaving the hatch covers off when in storage, let's the compartments breath and dry out.

http://www.noahsboatbuilding.com/noahmain/itemdesc.asp?ic=MDR%2D420&eq=&Tp=

yep  - "forget" the hatches and use inspection ports in the bulkheads.

I was never planning on using the hatches on my boat and yesterday actually pulled out the hatch material from the kit - not only will I not have leak issues, I will save around 2.5kg in finished weight.