I started by taking two treated 8' 2x6's and cutting 30" off of the end of each one, leaving me with two 30" and two 66" pieces. These were then stood on edge and screwed together to create a simple rectangular frame 5.5 inches tall. I could have gotten away with using 2x4's but was already picking up a supply of the 2x6's for another project. That part took all of 10 minutes
Then I cut a piece off a roll of 10' wide 6 mil black plastic. I made sure it was generously wide and, of course, it was way too long for the length of my frame. The plastic was laid into the frame then filled with an inch and a half or so of water to settle it into place and hold it down.
The plastic was then carefully cut to size and fastened in place with some 3/8th thick strapping.
By draping the plastic over the frame and trimming it off about where it reached the deck, there was enough of it to get a turn and a half around the strapping.
To keep things under control while I was winding the plastic around the strapping I used spring clamps, taking care not to rip the plastic with them. I also made sure I didn't pull at the plastic while getting the strapping in place but rather just let it lay inside the frame as it wanted to.
Then, using the same stainless screws with gasket-ed washers that I used on the roof, I fastened the strapping to the top edge of the frame.
All that was left was to trim off the excess plastic at the corners
and load up the tray with pots.
As with the grow boxes, as long as I top up the reservoir when it gets low, which seems to be about every two to three weeks, the whole setup is self watering. I also have some control over the moisture level in the pots with the water depth in the tray. Too dry I just raise the water level, too moist I lower it.
I thought about getting fancy and installing a drain in the tray with one of those two-part fixtures used in the liners of above-ground pools, but decided that since I already own a couple 12V pumps and the plumbing to go with them, I can just use one of them to pump water out of the tray if it's too full or I need to empty it to move the whole thing.