In case any of you have never read the extraordinary "Sliver" thread on SA, you really should, but you'll need two full days - it's now more than 5000 posts.
Here is a link to a post that begins a discussion of mating a hatch to sloping surface on the newly-build Sliver. The same process, or one you closely approximate would work for you. To make it easy for the DIY-er I might hybridize what I have suggested above, and this.
Perry Sliver Class Day Sailor - Page 51 - Cruising Anarchy - Sailing Anarchy Forums