I haven't covered our happijac seats yet, but in our motorhome that we had until a few months ago, I sewed very simple covers from old quilts I bought at the thrift store. They weren't perfect, but they protected that RV well for 10 years. When we sold it the upholstery looked brand new and it had a lot of use.
I will buy either quilted fabric or new quilts from Ross or somewhere else cheap so that I can them all matching this time! Right now I'm just using fitted sheets to cover the couch and beach towels for the happijac seats. I bought elastic sheet clips from Amazon to keep the towels in place and so far they work great!
https://www.amazon.com/Adjustable-Tr...NsaWNrPXRydWU=