Matt,
Here are a couple trailers we use when hunting. The first is a half top that might work well with your mule depending on how tall it is. BTW that picture is about 1 mile from where we hunted yesterday. Looks like a different part of the state though. East Texas piney woods, vs rolling hills with hardwoods. Seriously 1 mile apart.
This is the stock trailer that Mandi and I use the most. It is a 24x6 with two cut gates and a 4x5 escape in front. holds an easy 8 dogs chained in front. plenty of hogs in the middle, and 3 horses saddled in the rear. Or as a side job it will haul 12 head of heavy bred brangus PACKED in there.
Brute, I too like your trailer setup. I had a hitch put on the rear of my new trailer just for the purpose of hauling a little trailer like that in tandem.