When using the STLINKV3 with the additioal connector board MB!440 the original back cover can no longer be used. I have designed a 3d-printable back cover that fits the STLINK when the board is attached. I have also made a little connector board that allows me to connect a 6-pin Tag-Connect debugger probe to the JTAG connector.

The 3mf file and the Gerber files can be found on my github page:
github.com/olafdreyer/stlinkv3mod
Cheers
O.