I tried big dropper, long-stem plastic bottle, short-stem plastic bottle, syringe and the little glass bottles/dropper that came with my JC sample kit.
For the DSE901, I liked the short-stem plastic bottle. For the M401, I like the little glass bottle (=~ 3 ml) and dropper from JC. Looks like you can also get the little guy from Midwest Vapor. I don't know whether the glass is as thick as for the JC bottle.
I can't imagine anything as convenient as a little clear dropper for getting down in a cartridge and delivering measured drops. The bottle doesn't create a very big bulge in my pants pocket. Unfortunately
Dave W.