I do appreciate your reply to my post. I have contacted Invacare and one of their distributors and found that the first item is actually the support for the wheelchair seat which screws or bolts onto the wheelchair rails and forms a floor that su;pports the wheelchair seat. The second item would not work either.
What I am looking for is a waterproof fabric with ties on the corners that I can lay on the wheelchair cushion and tie to the wheelchair frame so that it can be removed in case there is diarrhea or urine that escapes and gets on the seat. Even if I have only the cover itself, I could put grommets in the corners to serve as secure holes in which to install ties to the same end: namely to make it easy to remove,clean,dry and tie back on without having to dissemble the wheelchair seat and cover which in many cases is a hard job, and not to be undelicate about it, you are unlikely to ever get the smell out of the cushion. I have contacted Sumrise Medical and they don't have such a product. I can't believe no one makes such a protective cover. But I can't find such a product as yet. As an aside, It sounds like a business opportunity for someone who can sew at home and offer the product on the Internet.