Need help importing/mapping a multi-texture block model

I'm currently making a mod for 1.12, and I was trying to add some glass panes. Unfortunately, the built-in block base doesn't work on older versions of MCreator (I'm using 2020.5), so I had to make my own model. It uses 3 textures (top/bottom, left/right, front/back), but only 2 mapping options are showing up. How do I properly do this?