Here is a low cost and robust way to make an air tight cap for any refrigerant valve. I didn't have a blanking cap so of course I made my own. All you need is a appropriate sized flare nut.
Step 1: The Tools.
A propane torch and electronics solder. A high wattage soldering iron can also work.
Step 2: Making the Seal.
I used a non stick base to rest the nut. My aluminum step ladder was perfect for this. A ceramic tile or stainless steel plate will also work. I couldn't show myself applying heat and feeding solder into the base of the nut.
Once the solder melts and bonds onto the base of the nut, just let it cool. Now it is ready for installation.
Step 3: Installing.
I used a Ratchet to drive in the nut. I used a spanner, not pictured, to hold the body of the valve.
Now the valve is capped and hermetically sealed if ever the valve is accidentally opened.