I knew it would be pretty much impossible to find something pre-made in the size I needed, so I decided early on that I would make something. I picked up a 28"x30" sheet of plexiglass at Lowe's and started winging it.
I've never really worked with plexiglass before, and it wasn't too terrible. After measuring the planter to determine what size lid I needed, I got to cutting. I used a straight edge and utility knife to score the plexi, then placed it on the edge of a step outside and gave it a sharp hit. Sometimes it broke cleanly and sometimes my score wasn't deep enough and I had to go back with pliers to break off the rest.
Once I got all my pieces cut, it was time for assembly. I attached the two long sides to the top first.
I put together this lovely little set up. I placed the whole thing on that wood circle (for another project to come!) so that I wouldn't get any adhesive on the table. I used the Intel box and the basket to keep each side piece propped up.
I tried using super glue first, which really didn't work at all, so I switched over to clear silicone. I just ran a bead along the entire seam and then smoothed it with my finger.
I let those three pieces dry, then attached the two short sides. I used making tape to hold those pieces in place, then applied the silicone. I flipped it over at this point to finish drying because some silicone had squished out and was trying to stick to the wood.
I let it dry until the next afternoon and then removed the masking tape and placed it on top my planter.