Thank you again!
I ordered 4' x 250' roll of the row cover in black and white from place above.
It came well packaged in a 4' long box.
It was easy to unroll, and I cut a 3' length, so I had a piece about 4' x 3'.
I then cut that in half, so each piece was 2' x 3'.
I bought Alliance 17" rubber bands off Amazon to secure them to the Earthbox.
The 17" bands are the perfect size, and tightly secure the row cover under the earth box lip.
It was super easy, and they are very secure.
Great solution!