A case where you can store your Print-Cores in and that you can hang on the side of your Ultimaker 3.

I now adjusted everything to a more compact size where you have to click your print-core into the print core holder just like the print head.

(An idea I was asked to draw and it works! When you have multiple Ultimaker 3's and you have 2 build Print-Cores + 1 support Print-Core for every printer they tend to get used for other printers. The problem with this is that every single time your UM3 printhead registers a different Print-Core it asks for a X / Y calibration before printing a 3D model. You have the option ignore, but it is less frustrating and time-saving to use the Print-Cores that one specific printer already has registered so the proposal of doing the X / Y calibration doesn't show up.)