Many will be familiar with the inadequacy of the standard shock cord deck fittings on sea kayaks. Surf + shock cord = lost water bottles, sponges and so on. I found that netting was also not always up to the job in surf. One can purchase deck bags, but I decided that I would try and make one.
The materials I used are as follows: shade cloth (from a hardware store), denim cloth for a closure flap (I happened to have an old pair of jeans), velcro, shock cord, olive cleats (from a marine store), lightweight nylon cloth for edging, brass eyelets, heavy nylon cloth (for reinforcing around eyelets).
I began by measuring the area on the deck where I wanted the bag to be. I then cut the shade cloth slightly larger than this area. I cut the top of the bag about 5cm larger than the bottom so that it easier to access the contents. However, access is quite tight, and I would leave more room if I make another one. You could also try gussetting at the sides as Jackie Windh has done. This gives you far easier access to the bag, but it also gives your bag a higher profile on deck. I made a large closure flap from a piece of denim cloth, doubled over, glued and stitched, and sewed velcro onto it. I sewed this to the base of the bag, and then sewed the opposing velcro to the top of the bag. I then double stitched up the sides of the bag. I sewed the edging on to cover up the rough edge of the shade cloth, and had the eyelets and reinforcing cloth put in at a camping store. The bag is secured to the deck by olive cleats and shock cord. This set-up has so far withstood waves breaking on deck and a couple of surf capsizes.
To reduce the chance of losing the contents of the bag should the velcro fail (quite possible), I run the deck shock cord across the neck of the bag with an olive cleat connecting the ends of the shock cord. With this arrangement I can tighten the shock cord if going into surf or rough seas, and loosen it off for easier access at other times.