Trivial to make one - you just need a SDR and encryption key (harder to obtain, but probably could be found on the black market).
That’s why it would be good to shut down GSM at some point: It would raise the difficulty of such attacks significantly.
What I don’t understand is how they managed to actually intercept any SMS with an IMSI catcher. They’d need to get the network to send these through their infrastructure, so I wonder how that worked?
Update: Ah, they were just sending out texts themselves, not intercepting anything.