Elastic deck net is, I think, hard to come by. I've always bought mine from a shop in Brest, France. The net is quite cheap, but the postage expensive for a low value item. I'm going to order some more for myself and would be happy to order extra for anyone who is prepared to COMMIT to purchasing it. Price will be what I pay, plus an even share of the postage. Here's a picture / price:
Filet de pont pour kayak de mer
It's sold in either 0.5m or 1m length and fixed width. Measure across the width of your deck, between the deck lines, where you want to install the net. Mine needs about 60cm, ie. a 1m length, just in front of the cockpit, but 0.5m might be sufficient further forward and behind the cockpit. You'll need to cut to size with scissors, heat seal the cut ends with a lighter / gas hob then untie your deck elastic / lines and thread them through the net to fit. See website picture.