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
Hey Peg I was Looking at the Cross inserts too! Might give them a go when the jetstream runs out, its actually become my main pen because of how smooth it writes and how it feels in the multi drive. As I spend more time with my multi drive, I came to the realization if you guys do another manufacturing run of the Stubster, making the handle about 2.5 cm longer, internally would let it accept the pencil/grease leads, and pen inserts that fit the collet! As it is, it’s just a liiittle too short for that and you have to break the lead in half. It’s also bit small for my longer finger hands and I end up reaching for the full size 
. That or maybe an in-between size in something like PEEK.
Edited as the original comment didn’t post as a reply.
Love this combo - My Multi-Drive is primarily a pen now for me, but love how many options I have now. Thank you for this post!
