I've had these shared for a few years now in previous Forza titles and have imported them into Horizon 3 for everyone to use. I have the standard logo and one of just the bomb. Here they are on a Trophy Truck I dolled up. I believe the easiest way to find them is to just search my Gamertag, it's the same as my username.
I'd love to see what designs you guys come up with using them :)