you could anchor it with rebar stakes such as those in the NHZ 16-inch J-hook ground anchors 8-pack or attach large screws to the back wall of the planter box and balance the trellis on top of them.

We also appreciate these features: The optional trellis supports ... Secure the lattice panel to the back by driving screws through the panel legs into the planter box. Screw shelf boards to ...

Down to even the same dimensions, this planter has a solid wood box and trellis behind it. Arlmont & Co. Wooden Raised Garden Bed with Diamond Lattice Trellis, $114.99 (originally $123.99 ...