Just figured I would pass this along, but I found a high quality pen insert that works perfectly with the multi drive.

These colour pencil leads also work great for those of you looking for coloured options

Just wanted to pass on my findings as I see what all I can use with it
Note that the stubster is not quite long enough to fit the pen its about 2cm to short :(
Hi, thanks for sharing, this is a great find! During our prototyping and testing phase we also found a pen cartridge that worked - It's a Cross Medium 8513