Here is a link to a Flickr album showing how I made several of those
https://www.flickr.com/photos/girer/sets/72157634814698407

The basic design, which I found on this forum almost ten years ago, is to glue two components together to form the waterproof finger hole. The bottom component (2"L x 1"W x 3/4"Thk gets a groove routed into in with a core box bit. The top component is the same length and width, but only 1/4" thick, and gets a hole through it, the same diameter as the width of the groove in the bottom piece. The hole lines up with one end of the groove. I epoxied both pieces for waterproofing, then attached them together with thickened epoxy. To use it, drill a hole through the hatch cover (same diameter as in the block), then attach the block to the underside of the hatch cover, with the hole in the block aligned with the hole in the hatch cover. Size the hole and groove to fit your finger. Obviously your finger can then fit into the hole, under the hatch cover along the groove, and pull upward on the hatch cover to remove it.

As the photos show, I made several of these blocks in larger pieces, then cut them to size after the epoxying and assembling steps. Then I rounded the edges and corners, then epoxied the outside surfaces. Water can, and does, get into the holes, but it cannot get into the hatches. When I put the kayak onto the car rack the water runs right out of the blocks.