To save money (and I had no idea were to buy elastic in St. Maarten) I just cut the elastic out of the fitted sheet and reused it for both cushions. I had the flat sheet for one cushion and once I cut the elastic out of the fitted sheet, I had a flat bit of fabric for the other. I didn't have any idea how to make fitted sheets, but I Googled it and it seemed pretty straight forward. I just draped the fabric over the cushions, cut it to size, cut out the corners, hemmed the straight edges, pinned & sewed the corners, and then sewed the elastic to the corners.
Here's the old with the new:
And the final product: