I expected Home Depot to suggest the appropriate fasteners on the page for the coroplast, but it either suggested other coroplast or completely different products. When I searched someone suggested
#14 x 1 in. Self-Drilling Screw with Neoprene Washer 1 lb.-Box (62-Pack), but even though i specified coroplast, that, like all of the other results I found, was for those long and narrow single-layer fiberglass sheets with corrugations inches wide, not coroplast.
In the questions and answers someone wrote "Can these withstand being in direct sunlight for extended periods of time?"
"We would not recommend this product for this application. Thank you."
Where else would you use them?!
I did find out that this is how you are supposed to join panels:
2 in. x 96 in. x 1/4 in. Thermoclear Polycarbonate Multi-Wall H-Channel
Since I didn't level the garden box or cut the studs to match its angle it leans. I had wanted to make braces from the metal shelves that I pulled out of the garage, but I was also thinking of using braces like these to secure the studs and fix the lean:
1-1/2 in. Zinc-Plated Corner Brace Value Pack (20-Pack).
I just need to bend them slightly.