Most folks (in my experience) would call this a day hatch. Many people mount VCP hatches from the underside of the deck with epoxy. Check that the distance from the lower edge of the cover to the flange of the rim is greater than your deck thickness! If your deck is essentially flat where you want the hatch and you haven't put both bulkheads in for the dayhatch compartment, this should be easy. Cut a round hole in the deck and trial fit the rim. Then, apply thickened epoxy, slide it into place and clamp. If your deck is arched enough to interfere with the cover, you will need to use a recessed hatch detail to back-mount the rim. This will also allow you to backmount the rim if you have already put in both bulkheads. The oval VCP hatches can be back-mounted after the deck and hatches are installed but the round ones can't.

I have seen one top-mount detail on an arched-deck CLC kayak where the builder carefully shaped the back of a disk of wood to match the deck arch. The VCP hatch rim was then top-mounted on these adapter disks on top of the decks, leaving them sitting VERY proud of the decks. I think I would choose not to use this detail.

Kayaksport (www.gokajaksport.com) also makes 'VCP'-type hatches in 6 sizes including 2 ovals and 4 round sizes. The smallest is 4" in diameter, making it useful for an inspection hatch or a hand hole near the end of the kayak for loading and unloading the area beyond the skeg (or loosening stuck drybags).

: A saddle hatch is a third water tight compartment just behind the cocpit. It
: allows you to have secure storage that you can acess on the open ocean
: without the risk of broching waves flooding your bow or stern
: compartments.