One way insert for the tonal shelf from TonalShelf.com. The drill hole that they do allows the t-lock to spin with no hard stop. My wife kept putting accessories on where they weren't sitting in the groove at the bottom. This caused accessories to drop to the floor.

The shelf uses 22.13mm wide by 22.30mm deep holes which this will fit in to make a twist right to lock stop and twist left to release stop just like the machine.
On my 6 accessory shelf two of the holes are shallow due to the shelf mounting bar that goes into the shelf. I've included the shallow 12.30mm deep model as well. Check the depth of your holes and adjust as necessary.

Prusa

No

.20