I've been making and using a lot of sampled loops lately— it's a fun way to practice. And this is a fun one: the main riff from The Wizard, from Black Sabbath's first record. It's doom-y in kind of a hilarious way. Actually playing the riff is fine, but you can run anything you want with it.
I suggest getting one of those allegedly-legal YouTube audio ripping browser extensions/add-ons/plug-ins and ripping the mp3, and setting it to loop on your mp3 player. It loops cleanly, so you can play all day without resetting it.