Personally, I think of puñal as a “dagger”, while the words “shank” and “shiv” have much more slang connotations, but it’s pretty close. I think of navaja as more of a pocketknife, but as far as I understand shanks, shivs, or whatever are pretty small (although my only knowledge of such things comes from watching the CourtTV channel, so what the hell do i know) but I think either of them would work, you just lose the prison connotation in the translation. Maybe if some flutherers show up who have been in jail in Mexico, we’ll all learn a more specific word for it.