I too have dabbled with a goose quill on the Appalachian dulcimer, but found the grip quite a strain on the right hand and the flaky bits rather messy.

Having read of Jean Ritchie's preference for margarine tub lids, I was inspired by this queen of folk. I have experimented with a variety of materials and can recommend Carte D'Or ice cream tub lids as having a perfect rigidity for cutting into outsized picks for noter and drone style playing of the dulcimer.

Maybe that could work for the cittern too?