How to mention new offers after interview

I recently finished final round interviews with a few banks (think GS/MS/Moelis/Evercore) and am still waiting to hear back from them. A few days after my final rounds ended, I got an offer from an elite boutique.

I would still prefer to go to one of the other banks, though, so I want to somehow tell them about the new offer in order to seem like a strong candidate. Is there any way I can mention the new offer to the other banks without sounding like an arrogant a**hole? I have a few months to decide whether to accept, so I can't use time pressure as an excuse to mention it.

Any ideas or thoughts would be really appreciated. Thanks, guys.