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And they are easily found if you look at places that handle transitional or sea kayaks. No need to make your own for a known cockpit size like this unless you want to save some bucks. I am not sure how much you save though.
To avoid losing them driving, just get a basic carbineer and clip the loop (they will still have loops) to the bungie or other nearby deck rigging. If that is missing run a strap around the boat and clip it to that. A two dollar investment clip will save a $60 cockpit cover.
That said, we find that the neoprene covers come off less easily driving than the nylon ones. I can't get off of one hand on counting times we have looked in the mirror and seen them flapping around.
The value of a cockpit cover is that it reduces wind drag and just generally keeps things more secure driving. At home it protects against things like sun damage or critters eating part of your bulkhead. Squirrels can be fond of nesting in that nice foam bulkhead that will be a PITA to replace if you have to reshape and fit one yourself.
I'd consider it part of your basic boat gear unless you have inside storage and tolerance for a pretty noisy ride to the launch site.