: Anyone out there with a downloadable pattern for a nylon spray deck that a
: person with reasonable sewing skills can make up at home?

You already have the one and only good pattern for your boat: the coaming on the boat!

Use that and you will have a fit that won't sag. Sit in the boat and figure out where your figure puts the funnel so it won't chafe. Measure the height from side of coaming to armpit and you have the height of funnel.

Ask yourself if you want to be able to acces the rear and front of cockpit without opening the skirt, and leave room for your hand if the answer is "yes".

etc.

Making your own means you will get a good one. Standards only fit certain boats that are mass-produced, and rarely people.

Shawn gives good advice on his site.