If you use a leaner meat then soak your bread in heavy cream for added fat.
These components help the meat stick together in conjunction with the fat from the meat and its natural moisture.
If you plan to use a different type of meat then be prepared to make up for the lack of fat by using cream, extra eggs, and perhaps even mixing in another kind of high fat meat.
Some suggestions include barbecue sauce, ketchup, chutney, mushroom sauce, gravy, vodka sauce, marinara, Alfredo sauce, or even steak sauce.