So I traced them onto some new home decor weight canvas from Ikea I had and added a seam allowance. Then I made some ties from an old pair of jeans cut into strips (I made 8 and folded each in half at each corner). I stitched around the curved edge, then across the straight edge with the straps inside, and left a hole for turning (and stuffing) in the middle of the straight edge. Then I hemmed the opening on each side, and intended to put snaps there.
Then I turned them and stuffed them (I cut the broken ties off the old cushions before stuffing). I put them onto that chairs without adding the snaps, and think I could leave the snaps off for a while. Let's see how the troops like them first.