I like the tattoo idea. They will get them anyway, so...what’s one more?
Tattoos are not enough. They are too easy to modify. 2 missing nonadjacent fingers is much harder to camouflage. A tattoo alone would be copied by all the Left to render it useless. I don’t think that there are a whole lot of leftists who are willing to give up two fingers for the cause of hiding the illegal felons. Non adjacent fingers because accidents take adjacent digits if more than one.