Hatches & Storage
This is for sit-on-top kayaks with closed hulls. Open-hull sit-inside kayaks don't really need hatches.
You can order any kind and size of hatch on eBay. They are easy to install - just trace the hole on the hull and cut it out with a jigsaw. A hatch like this one has a storage bag for small items like keys and phone, which removes for access to the inside of the hull.
Sit-on-top kayaks generally have a hatch located in front of the seat where it is easily accessed. The hatch usually just opens into the entire hull, which is not very useful. Imagine your car keys all the way up at the bow. There are several things you can do to remedy this.
The simplest thing is to tie a lanyard inside the hatch that you can clip things to. A shoelace works great. A better solution is to make yourself a proper glove compartment. This is actually easy:
Get yourself a Tupperware container like the one above. That one is 14 cups, about 12 inches square; there is also a flatter 9 cup model if the bigger one won't fit. This is about $15 at any supermarket.
This container is bigger than the hatch hole, but you can fold it and push it through. Drill holes in the corners below the lip, and tie it inside the hull with a long shoelace, bungie cord, or whatever you have, as shown below. I used bungie cord so I can shove the box aside and access the rest of the hull.
Add a couple of cheap carabiners, and you have a perfect place for keys, phone, spare parts, rope, small towel, hat, water bottle, etc. If any water gets in the hull, the Tupperware will keep your things dry. I tie everything down, especially the small things, so they stay put if you flip the boat over, like on a roof rack. The Tupperware lid is obviously unusable, throw it away.
If your kayak only has one hatch in the center, you should install a second one at the bow or the stern. That way, if something does get loose inside the hull, you can stand the kayak on end and give it a shake, then retrieve your item through the second hatch. Just try shaking your car keys to the middle of the boat!
That same Tupperware would also be useful as dry storage in a sit-inside kayak, but you can probably come up with something cheaper or free.