After some looking around, it appears that what we need is an "edge trim". Searching the Internet, we find a company called "Trim-lok" that makes such things, and has a large variety of them. We think the steel sheets are about .025 inches thick, which leads us to wanting the 750B2X1/32 edge trim.
But all the documentation discusses a straight edge, not a corrugated (or wavy) edge. Luckily, Trim-Lok is willing to send us a sample, and trying to install the sample on the corrugated steel sheets shows that it works fine. Trim-Lok sells it in 500 foot rolls, but we only need 26 feet (the circumference of the 65 inch by 90 inch raised bed). But they point me to Grainger who carry the 750B2X1/32 edge trim in 25 foot lengths ($34.21).
Then it's down to installing it. This took more work than I thought.
Mostly it slips on pretty well -- the plastic body has enough flexibility to bend first one way and then the other, back and forth following the curves of the edge of the corrugated sheet metal. It was a bit like weaving. Position the edge trim over the sheet metal and then push down to insert the metal edge into the edge trim.
The hardest part was where two sheets overlapped. At these sections, we have two sheets of sheet metal, but the edge trim is wide enough to just barely fit over them. We used C-clamps to hold the metal together while putting the edge trim over them.
We first did the back wall, then the East side, the front and the West side.
And once the trim was all in place, we could fill the raised garden right to the brim with dirt.
