Last year, I had pretty good luck growing cucumbers in one of the rear containers. Unfortunately, my flimsy bamboo trellis blew over in a thunderstorm and I lost several weeks of cucumbers.
Clearly I need a far more substantial trellis so I decided to put an arched arbor between the two rear containers. I found an inexpensive arbor that is almost the exact height and width as the entry arbor at the front of the garden. Life is good :-)
I built a platform out of pressure treated 2x4's, got it level and screwed it into the adjacent containers. The new arbor is screwed into the platform and the yard box is now level, as well (YAY!)