I found a quick, cheap, and easy divider solution I'm pretty happy with:
All you need is one medium / small bungee cord and one heavy duty ziptie. I hooked the bungee to the top rear PVC, brought it under the bottom front side, then looped it back to the top rear. The bottom front edge holds tight, but the rear top needs a bit of added reinforcement via ziptie - Just tighten it around bare PVC and the hooks won't slide past it. Everything is still adjustable, but it'll stay in place once it's fitted to your liking.