We’ve just received the 3.6kg cartridge. Turns out that it sort of fits, but a couple of modifications are needed. The 3.6kg cartridge does not depress the cartridge limit switch just above the needle, so some sort of snap-on cartridge extender could be needed to make sure the sensor switch is pressed.
Also, the shape of the plastic cladding panel around the material cartridge bay interferes with the shape of the 3.6kg cartridge. The 2kg cartridges are more elliptical in cross section, so the more rectangular 3.6kg cartridges need to be inserted slightly askew. And the depth of the cartridge is about 5cm longer than the 2kg cartidge, so material bay door must be left open.