Uninsane

I have a Franchi 500 from the 90s (imported by Stoeger) that I'd like to use for 3-Gun. The current mag capacity is 4. Numrich gunpartscorp lists a magazine extension tube for this but nobody stocks it or makes it anymore. Does anybody know where one might get this extension tube? If not, does anybody know if the threads and depth of the nut matches any more popular firearms? Has anyone every fabricated a tube of their own? Thanks in advance for any wisdom you can dispense!

I'm a 3 gunner and trapshooter and I'm interested in learning more.