Q:
Universal Multitool. What do you put on it?
A:
In order to socket the backpack I ended up having to transmog its look to another piece. Then and only then was I able to socket it. Give that a try.
I chose the monocle so I can’t say for certain but I think lvl 1 gems (the ones you get from copper) might fit it because the mid level back pieces from the story quests and the halloween event back pieces had the same problem.
I’m sure that works; if you transmute, then the upgrade slot is coming from the other item so would not be having the same problem as the Multitool. It does sound like you are confirming this issue occurs, so I will report it as a bug.