I generally write out a handwritten certificate with the date on it commemorating the event...I actually did that once. We used to have these "First Flight" certificates, and we'd fill them out with our names and the flight number, etc.
I just used one of those, and changed a few words around to include their newly attained MHC status. I guess things like that just don't bother me. I'd prefer that MHC status be attained in the can, as opposed to out in the cabin...now that's something I wouldn't care to deal with, and a few of my friends have had to "break things up" when it got a little too rambunctious. I was on the flight, but in a different cabin. Funny thing was...her boyfriend was on the flight with her, but he was not the "object of her affection"...so to speak.