I've done it a bunch, it's no big deal on the aft deck where there isn't much bend. Cut hole, install curved stiffener under hatch, install spacer and flange under deck. The spacer doesn't have to be wider than 1". Round over one edge of the spacer and flange so that stacked flange and spacer have a smooth transition with no sharp edges. I prefer swivel enclosures with one strap providing a quick clasp and tether for hatch.

The front hatch is more problematic because there's no way you can maintain the curve of the hatch beyond where the stiffener holds the curve, so the last 1" beyond the stiffener tends to flare out about 2mm on either side.

If you haven't put the deck on I'd suggest pre-glassing the underside of the aft deck from the bulkhead to the far aft edge of the hatch hole with two layers of 4oz glass or one layer of 6oz between hatch and bulkhead. One layer of 4oz for the foreward hatch. You'll add all of a few oz but it'll make a significant difference for the hatch and especially the open section of the aft deck between hatch and coaming.

Heck,,get some 3.25oz tight weave glass from RAKA for the underside and you won't add any weight compared to two sloppy sealing coats.

When you make the hatches out of the cut deck round the edges off nicely and add about four coats of epoxy slightly thickened with cabosil to the edges. The edges of ply can break through pretty easily and once water (like standing water in a flush hatch) hits dry edge grain ply it's soaking in FAST providing a nice tell-tale black mark. Also it does't take much effort to sand right through two sealing coats on the edge of a 4mm piece of ply.