I made this simple model to fill up the space above the Supernote pen refill in a Lamy Safari Rollerball body. My refill snapped into the front of the body with an audible “pop!” and stayed put, but rattled back and forth a little. This spacer holds the body centered, and fills the space behind it to keep it solid and steady. I have included a picture to show how much the refill tip protrudes beyond the Safari body. I have only tested this once, so ymmv.
The design is public on OnShape, and can be copied and edited there: https://cad.onshape.com/documents/29e0edbf30b7f525c404a821/w/199fe78f80980c8ed5c2945d/e/71eaef0283a15674ad3a038b?renderMode=6&uiState=663b9650d7c95a0974bf8341
Recommend printing with at least a 3mm brim to aid bed adhesion.
The author marked this model as their own original creation.