We had the cushions recovered in the Trill a few years ago. I don't remember what it cost, but it wasn't bad. We used a tapestry on one side and a plain color on the other, so we can mix n match.
I have been a seamstress for 45 years, but didn't have time to tackle the cushions. I did make the curtains & lined them with black-out fabric to block the light
from the all-night lights
in campgrounds. It works very well.
I do want to say: Unless your fabric has been pre-shrunk before cutting, DO NOT remove it from the cushions and wash it. The zippers are there for the convenience of the manufacturer in getting the cushions put together. If they are made of upholstery fabric, they may not even be washable: they may distort, shrink and/or shred in washing. You will not be able to get them back on the foam.
It is best to surface clean them, using a foaming product made for that purpose, or make a thick foam from soap and use that. Rinse gently.
Ask your provider what the fiber content is, and how he/she recommends cleaning it.
If you are making your own covers, one trick to getting them on the foam is to wrap a piece of heavy duty Saran Wrap type plastic around the foam so they slide in. Then pull the plastic out.
I once made all the cushions for a 50'Grand Banks Trawler - one side fabric, the other Naugahyde, all different shapes & sizes. Boy, was that fun!! NOT! <_<
Good luck with your projects.