There still isn't an official API for custom Android Wear watch faces, but there are plenty of them in the Play Store anyway. If none of those strike your fancy, perhaps because they don't account for the taller peek cards, Facer could be just what you want. This app lets you build and edit watch faces and sync them over to the watch in a snap. There's also a fast-growing community of users posting some really awesome designs.
Facer allows you to specify images and screen elements for your watch face using a WYSIWYG interface. You can download watch faces from the built-in gallery or from folks on the internet. They can be edited as you like pretty easily too. Building your own watch face in Facer is a little complicated, but there are tutorials on it. It's a little like Canvas for Pebble.
You'll need to be willing to manage a bunch of ZIP files and folders to start tinkering with Facer, but the results can be mega-cool. For example, the mountain face below works with the sensor bar at the bottom to form a mountain outline that helps hide the "flat tire" shape of the screen. You can also easily move elements out of the way of those taller cards.
Of course, Facer isn't perfect without the final Wear API—there will be some weirdness with many of the faces you download. I'm a little embarrassed by how many compromises I would be willing to make for a Fallout watch face, though.