Did these with a Noodler's Ahab filled with Daler Rowney Calli ink. It's waterproof non-clogging india ink. I know you aren't supposed to put india ink into fountain pens. But since the Noodler's ahab can be fully dismantled and washed, there is no fear of permanent clogging. Also I try to wash it every few days or so since india ink could be a little corrosive to the pen feed. But it works well for a waterproof ink in a flex pen! And the Ahab has a pretty decent flex for drawing (not too soft and not too stiff).I added tape in some of these just for fun.
Noodler's Ahab flex pen + watercolour + chinese ink.