Well, I am not sure what the best thing for you to try would be but if you do decide to remove the cover off the cushion and wash it and then replace it, I would suggest putting it back on the cushion while it is still wet so it will dry stretched to the perfect size. Set it outside to air dry. I have done this with my denim cushions and even thrown them in the washing machine! It is quite a struggle to get them back on the cushions but then when I let them dry on the cushions, I have no problem with them getting stretched out or shrinking. Good luck!

Sounds to me like the cotton you would like to use will be fine. I would not hesitate using a medium weight cotton on a dining room seat chair. In the fabric description you provide, I see the word "duck" used. Every time I've felt cotton that has the word duck in it, I notice it feels stronger/thicker than other cottons. Good luck with your chair seats & post before and after pics so we can see! Laurie

I'd definitely split that 100" length up. If you are no fan of installing zippers (like I am), ties or buttons could be used instead. Velcro could be attached to the bottom or the back of the cushions and adhered to the wall or bottom of bench to keep them from slipping around. If you don't want to do box cushion covers, you certainly don't have to. I've made covers that are sewn like a pillowcase. Slide the foam in like a giant pillow, and center the seam along the length of it. Poke the corners in and do a hidden whip stitch to close them up neatly. Pretty simple, really. If you really, really want to avoid sewing(. . . I've done this, too! Grin) the simplest cover is to snugly wrap the cushion with your fabric of choice like you are wrapping a gift. Secure the fabric with large heavy duty safety pins where they won't show. This is easy to remove for laundering, too. Enjoy your window seat. It sounds lovely!

I recently had a very different experience than Scott with Cushion Source. I spent $300 for a custom window cushion with the piping and buttons. I was so excited when it arrived, only to find it had a very large pull through the fabric - it was the first thing anyone could see when they looked at it, and I am very surprised it was sent out to a customer at all. The cushion was also 1/2" too large, depth-wise. I contacted them immediately and sent pictures; one week later, I received a brand new, perfect window cushion, with which I am very happy! Looks great and I am very pleased with the customer service I received!!