I spent months trying and found nothing that worked or that I liked. I order several and sent them back. They did not fit or looked like a sack on a pig.
If you find something let us know because my research found nothing that looked presentable.
There is a thread that I started if you want to search. Some did chime in but not with much that helped.
The search function is at the top of the page in the gold bar. I recommend using this before broadcasting for a answer. It is not as user friendly as it could be but there is a wealth of info out there and most things you can think of have already been asked and answered.
The seats in the E-450 are not Ford. They are something that Jayco had installed so they have no "common" design that the different cover makers can use. Therefore, they have to be custom made. I did find one that came close but it did not look good or fit well when I tried it on the seat.