This is a very specific shelf designed to hold the TP-Link USB 3.0 hub (https://www.amazon.com/TP-LINK-UH720-7-Port-Smart-Charging/dp/B00SCX6I8A/ref=sr_1_1).  I got tired of them (I have three) sitting on my desk/bench and taking up the space and the mess of cables.

The build does not require brims or bridges, but Prusa Slicer will complain about “thin fragile part”.  You can ignore that warning.  It builds fine on the MK4S.