I created this cap to keep dust and debris from building up in my pneumatic tools. My hope is to prevent scoring on the cylinder and rotor or damaging the vanes. This is for a Milton 1/4in NPT M-Style but should work for any fitting with a similar OD of 0.316in (8.03mm).

I used an o-ring to secure the cap and added a recess for a small magnet. This way I can attach the cap to the air tool holder or tool box once it has been removed. The hardware I used is linked below

I used SME o-ring R-04 from a kit from Amazon. It should be the same as a dash number 011 o-ring but this has not ben verified.

I used a magnet that was on hand and it worked well. The magnetic hold is light but it works for the temporary storage when I am using the air tool.

When I printed it in PETG I had to increase the scale by 102%. Otherwise the o-ring was very tight.

Hardware - (No Affiliation)
O-Ring Kit (R-04 Used) - https://www.amazon.com/gp/product/B07GSKKHQM/
O-Ring Dash Number 011 - https://www.mcmaster.com/9452K19/
Magnets - https://www.magcraft.com/magcraft-nsn0658
Air Tool Holder In Picture- https://www.amazon.com/gp/product/B078WHF27Z/