Matt,
Here's a picture of my complete rig. I just sold my smaller trailer and got the front trailer and am modifying it to load a second four wheeler on the front top. I would think you could build one just like it and make the center area a little wider to accomodate your mule. There are six dog boxes on the front, and the back compartment for hogs. The back doors slide open to load/unload the days catch! The front trailer itself is 18' long.
Also...I have a four compartment dog box that I built (4' square with two doors on each side) that I used to use on the front of my old trailer. It's too big to fit in the back of a truck, but would work good sitting on the front of a trailer with your mule loaded behind it. If your interested, email me.
