I couldn't find the "UI Box Prefab" settings on this page.
Here's a screenshot of the editor I'm viewing
I'm running Unity on Arch Linux, so I'm not 100% sure that this isn't just some weird UI issue where the option I need just isn't displaying or if I just missed a step along the way that could explain its absence. Thanks for your time.
Thanks! That was it. I remember setting that value earlier in the tutorial (or another one like it, because I remember the warning that pops up about how changing this value will affect other settings) but I must have forgotten to click Save before exiting.