Just wanted to update folks. I did end up finding success using the Sunlite quick release binder for the range of 40-50 mm.

The binder does keep the post secured in the frame, but there is a slight bit of play side-to-side. Wondering if this is due to the shim and narrower seatpost situation?

Wondering what the diameter seatpost we could get that would eliminate the need for the shim, and if folks think that would help keep it better secured?

Thanks again!