UPDATE: The Hella Compressor Horn Twin Tone 12 V 780/840Hz has been installed. Here are a few pics. The steps involved are:
1. Remove front driver wheel and tire.
2. Remove front inner wheel well liner, it's just a bunch of screws -- Philips, Torx T27(?) and 10mm bolts.
3. Unplug original horn and remove it, just one 10mm bolt.
4. Cut the stock wiring harness (black is negative, purple is positive) and add your wiring for the new compressor.
5. Find good places to mount the two new horns and the compressor keeping in mind you will have to route some air hoses and wiring.
The vintage Ferrari horns I got from eBay didn't work out so well (one didn't work and neither sounded very Ferrari-ish), but the bracket was perfect for these new horns. Basically, a $90 bracket. EDIT: Wanted to also mention that I'm glad this doesn't sound exactly like a Ferrari horn. It sounds very Italian and suits the 4C so much better than trying to copy that vintage Ferrari horn sound.
I'm pretty stoked about how it sounds and after being quoted over $400 to have it installed from a couple places, I'm glad I did it myself. This also forced me get my own floor jack finally (I feel like an actual adult now lol) I attached some pics with notes and here is what it sounds like.