Hey Ben, I had a similar thought regarding foot design for my current build which has a base the same size as yours (roughly). I thought about doing something similar for the verticals, I was going to split a large dowel so it would be half a cylinder on the side of the speaker. Instead I decided to go with Herbies giant fat sliders (4 of them under each speaker). If you can wait for what hopefully won't be more than a few more weeks, I can report back if this is solid enough to work. I would think that a 16" square platform using 4 points of contact to the floor would give enough stability, even on carpet, but I won't know for certain for a few weeks yet.
I use Herbies products on speaker to base connections as well as electronic components and like them enough to try the giant fat sliders.
Best,
Ed