I wouldn't recommend Fountains for this application, a roller ball with a quality cartridge/refill is easier to handle...!
Those pens have typically 2 bulges and a valley, as the fingers need to have that sort of support as there is not much "grip" on a arthritis sufferer hand(s).
I don't know if there is any info on the IAP library on this issue but I know a place where, a few people have produced pens for this purpose, with lots of info and pics attached, and that place is the Australian Wood-workers forum
Woodwork Forums
Do a search in there also, and see what you get..!
Good luck,
Cheers
George