I carry my company ID and cert in a plastic holder. As those are the most common asked items for jumpseating. My medical, FCC, and ATP cert #2 fit into a pocket in my passport holder. My CFI and UAS cert stay in my wallet.

Before you ask, yes I have two ATP cert cards. I have a R-ATP and it takes up two cards for all the limitations & restrictions. I might have three when my replacement comes with my new type.