This project was a breeze. Total time was <90 minutes!
All I needed was foam and fabric for this cushion; I had the buttons and velcro already. The dimensions of the bench are 18 x 36, so a yard of outdoor home furnishing fabric would do the trick! You want the outdoor type since it is a more durable textile and generally has a Scotch Guard finish or something similar to help it withstand the elements.
The foam is sold by the yard and it comes in different heights. I chose a simple one inch foam.
I cut the fabric to take advantage of a fold and eliminate the need for an additional seam. So, the front of the cushion is on the fold and I have two side seams and one seam the length of the cushion. I sewed the fabric right sides together using 1/2 inch seams; leaving about a 10 inch opening in the long seam. After I turned the fabric right side out, I stuffed the foam through and sewed my opening on the machine.
I then sewed three buttons across the top of the cushion all the way through the foam to the underside. All of these buttons were in my button collection (I have two mason jars full of odd buttons).
The last step was to hand sew two velcro strips to the underside of the cushions; about two inches long on each width of the cushion. I then hot glued the "mate" to the bench itself. My intent was to provide an anchor so the cushion didn't go flying away in a stiff breeze.
Let the BBQ season begin!