"Spray Skirt" ? the one you are wearing while out on the water, or the "cockpit cover" you use when the WD is on its cradle/ stand? The skirt should have an internal bungee type cord to hold it beneath the cockpit rim. The cover should also have a bungee type cord that you can draw up tighter as needed.

   The question is why do you want a spray skirt on a big cockpit double boat.?   There will be no rolling. Waves should not break over the bow of your WD on shore during a storm. (where you should be) Water on a big cockpit skirt that size usually caves in the skirt and it isn't usefull. Most useful sea kayak skirts are around 16" x 33" more or less. Your cockpit is something like, 84.5 x 20.5.  I know other solo rec boats that had skirt implosions with cockpits no longer than 56" .  A cockpit cover would also need something like battens or perhaps an inflated beach ball. 

Now however, they put covers on canoes and paddle rough water.  There should be a "rand" or bungee in the edge of the skirt to fit under the cockpit combing edge. There would also have to be collars for the paddlers to fit up through. There would also be something to prevent water from collasping the cover.  You'll likely need "battens",  like in a sail, to keep the cover up and arched to prevent puddling and collasping......several battens. The canoe expample above often uses snaps, straps, and tracks to tighten the covers.  

   I agree with Grumpy's comments above.  Skirts are rarely worn in boats with HUGE cockpits like the WDW.  Skirts are designed to be quickly pulled off the lip in the event that you capsize and have to exit the boat quickly.  You could add velcro or something else around the edge but that will make it harder to remove and increase the change of getting trapped in the boat.

I mean the skirt of this type:  https://www.amazon.com/Seals-Skirts-Splash-Deck-Black-Reg-1-2-4-2/dp/B075QPD6Q1

Splash deck might be the more correct term (?).

It is of no use against big waves, of course, but keeps the dripping water out and protects from the sun. Also has handy pockets. 

It does fit nicely on the WDD coaming, but because the cockpit is huge and oddly shaped, it wants to eventually slide off despite being tightly pulled.