This has been a bit of a problem for me. I make A LOT of pens from aluminum and/or stainless. They work quite well EXCEPT if you make click pens the refill spring needs to made from non-magnetic Stainless Steel or Aluminum. Additionally, Make sure to check the writing tip of your refill, too. Some Pilot refills use non ferrous tips, the cheapest OEM refills are non-magnetic....others, I'm not sure.
The SIMPLIEST MRI pen that I've made is a MODIFIED Berea Sierra Ballpoint Pen-CHR (Berea part number 0502/B-CHR-1). To make this ballpoint NON magnetic, simply REMOVE THE CLIP and change the refill spring to stainless steel. (Be sure to take a magnet with you to the hardware store, as SOME stainless springs are magnetic).
ALL SPRINGS I've found from pen supply houses ARE magnetic as are MOST, clips.
Respectfully submitted.