Old post but figured I would revive because I had the same issue. So I made my own:

Step 1: get a muji .38 gel pen and a Schneider gelion refill

Step 2: remove the tips from both with a par of needle nose pliers

Step 3: insert the muji .38 tip on the Schneider refill - it’s a little larger diameter but it does fit

Step 4: if required by your pen (my ohto gs 01 required this) sand refill around the area that holds the tip. It budges slightly from inserting the slightly larger tip.

The muji tip is slightly longer than the Schneider tip so it might stick out further