Here is a easy to print ergonomic pen with holder that will take a bog standard biro inner. Once the inner has ran out of ink you can pull the pen apart and re-use it with another biro. I thought about selling this design but then thought better of it as i could see this helping people with dexterity problems and would rather give back to the community. Feedback on this first version of the pen greatly received.
NOTE: The biro inner does need cutting down to length, whilst i have shown a photo of the dimension i used you may find it best to insert the biro into the end of the pen (and ensure it is pushed all the way in and the put the grip on the other end and the measure the gap left at the centre to ensure you get a accurate length for your print. I did find a couple inners did not work as the ink was to far up the shaft so i couldn't cut them short enough. Most did fit though.