For those who might be thinking of breaking out their sewing machine and making one of these, here are a few sewing details.
This is the outside of the back panel. It's got a zipper up top which attaches it to the top panel, and zippers on the sides which attach it to the side panels. There's velcro hook along the sides and top to close it to the side and top panels. There's a row of snaps along the bottom to attach it to the tub frame and two snaps up top for the loops to hold it when rolled up. The bottom seam with the snaps has 1" webbing sewed into the hem to strengthen it so the snaps will never come out.
This is the inside view. The loops for securing it when rolled up are attached to this panel, not to the top panel - I did the so when this panel is removed, there won't be any straps hanging down from the top panel. Along the bottom edge you can see the stake pocket, inside it is a garden stake (you can see the ends peeking out), which makes it easy to roll it up evenly and tightly. The stake slides out of the pocket.