The first thing I did was look around my garage for something to attach to the back. I found an old cabinet back panel that would be perfect! (And if I didn’t have one on hand, I’m sure these are inexpensive to buy.)
I measured the space between the table shelves and cut a piece to fit, using a jigsaw. The first cutout was a little crooked so I tried again. Second time it was perfect! The rough edges around the cuts were very easy to smooth out using my orbital sander (a sanding block would work too).