I didn't see this posted anywhere so I figured I'd share.

I took the Lee 50 bmg extension and cut the shell holder portion off. Then drilled it out and tapped it for 1/2-20". The stud that is in it for the picture was just my fixture for polishing it on my drill press. It is definitely cut crooked, don't worry I haven't made any punches for it yet.

It worked out great and only cost $17. I'll be making a smaller OD extension for a ejection system.