With the Lewmar
hatches, in the Ocean Range (at least), you can get 2 different versions. The difference being in the hatch's base, which affects how they're mounted.
One version of their Ocean hatches have a Flange on the frame, which protrudes down into the mounting hole. So that this version of the hatch requires a flat mounting surface, as well as a cutout in the deck
which exactly matches the hatch's size & shape.
The other version of the hatch doesn't have a Flange, so that the hatch only requires a flat surface for mounting. Not a flat surface, plus an exact cutout in the deck
to match the hatch's shape. So that mounting of this variant is simpler.
But, yes, finding a hatch which perfectly fits ones boat
can be a challenge. AKA a pain in the backside. Though as already stated by another member
, you can build/have built, a new wooden or composite base for the new hatch you choose to be mounted to.
And if you do so, often times people will also build a breakwater for the hatch into this new mounting base. To even include the facility to attach a storm cover/sun cover to it for the hatch.
The breakwater doing just as it's name implies. To prevent boarding waves from directly impacting the hatch's front & side, & especially the gaskets there. So that it stays more watertight.
Ditto on the functioning of storm covers, etc.
The other option is to find a replacement hatch which is slightly larger than your current
one, & enlarge the cutout in your deck to accommodate it. It's not ideal, but...