If I were an instructor, . . . traveling from state to state to train, . . . I'd send out an invitation to Wilson, Colt, SA, and anyone else who makes a full size railed gun, . . . telling them I needed an INERT frame and dust cover with their name and logo on it, . . . and tell them the purpose.
Add a scrap barrel and innards, . . . hot solder the whole thing so it doesn't move, . . . paint it dark blue, . . . you are in the chips.
When I took a USPSA RO course, the instructor had put together a dummy gun out of worn out or damaged 1911 parts he had accumulated over the years. He epoxied the barrel full and didn't install a firing pin. He could run a student through a course of fire and demonstrate checking the chamber, dropping the hammer, etc. without an operable firearm.