Dunno about the Hartco but I just bought a Mustang off ebay. It looked like a very nice cloth-covered seat cover. When it came it turned out to be a slip-on type with an elastic in a sewn hem around the bottom.
I tried it but it just didn't look professional - it looked just like a loose seat cover!
I solved the problem by unpicking the hem and removing the elastic. I then fixed it to the seat pan properly, with staples, in place of the original cover. It looks very nice now and I am pleased with it, but as a loose cover it just looked cheap.
I've just bought a complete Hartco seat setup with drivers rest, the passenger backrest is a cover for the original and if it's the same materials for the slip on covers then the fit and quality is superb.