Our team built an X-Drive base (picture attached). But we have no idea on how to program it. We tried the mecanum drive sample code and that kinda worked. The front is the open side.
You may want to look at the structure of the X drive you built before adding the rest of the robot. As you put weight on the robot, those brackets holding your wheels will start to bow out. It works great now, but as you build the rest of the robot, it will probably change for the worse.