why not use Velcro?

I thought about that. If I sewed it on then I wouldn't be able to reverse the cushion later if I had to. I didn't think the sticky velcro would adhere well to the fabric and hold up to the elements.

Why don't you just turn the cover inside out, stitch across the top the amt needed, an turn back again--leaving the excess inside for someday when you might pull the stitches out and have a bigger cover again?

I should have mentioned, there is a flap enclosure on the top.

So.....how about adjusting the bottom seam?

That would work. I was hoping not to do a lot of sewing, but I guess it is just a straight line across the bottom and should be easy.

I did wind up just shortening the cover and they look good. I had six and it took less than 30 minutes.