neoprene and shock cord, which involves the same approach you would need.
Now, first a decision. If you want a really long-lasting cover, you should get neoprene with nylon on two sides. However, it is quicker and easier to make a cover from nylon-one-side neoprene.
Your shock cord should be 1/8" to 3/16" in diameter. First step is to pull a length around below the hatch lip and cut it with enough extra so you can knot it. It is easier to knot it when it is off the hatch, striving to get a ring that will be tight but not super-tight.
Next, you lay a piece of neoprene over the hatch, smooth side >up< if you are going one-sided. Then pull the shock cord loop down over the neoprene, and adjust the neoprene so the tension over the hatch opening is even and not excessive.
Next you cut some strategic acute triangular slivers in the excess neoprene, and using your neoprene cement (from a dive shop is best), start glueing the neoprene tabs back over the neoprene covering the hatch opening. Whether you use one sided or two sided neoprene, be patient, follow instructions, and let it dry properly before pressing the tabs down. There is no way for you to apply pressure from the other side, so the glue procedure is crucial.
You will need only a slightly larger gap for the knot in the shock cord. When gluing is done, you carefully take the cover off, and then reverse it. Now all the glue tabs are underneath, and if your boat is like my Necky, the plastic cover will buckle down over the neoprene, applying a sealing pressure against the hatch opening.
I do not expect such a cover to be as leakproof as the factory stitched variety, but I hope it will serve with only minimal leaking.