My problem was a lack of understanding about how a screw joint actually works. It's not the threads biting into both pieces that secured everything in place. Rather, the strength of a screw joint comes from the threads pulling the screw through the bottom
"Easy-to-build A-frame trellis to support cucumbers, small melons, gourds and sugar snap peas. Plant crops that prefer cooler temperatures, including cilantro, lettuce and spinach in the space underneath the a-frame.