I would make at least two so they can be flipped/turned, etc, for even wearing. If one long cushion gets worn or flattened in the middle, the whole thing would need to be replaced. Also, I would apply the zipper at the end of the cushion(s), slightly around the corners, so the covers can be put on like a sock and there would be less scrunching of the cushion. A non-slip type of runner(the type that keeps rugs in place)can be placed under the cushions to prevent slipping. JMO, hope it helps.

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!