I did a harness a little while back (last summer) after getting pushed around on fairly flat ground with the 1025R by a 10x5 tandem dumper with a load of dirt.
The simplest approach is use a controller with a manual activation feature (I bought a Curt 5110 for about $30). You can "pinch" across the width of the controller to apply braking. You also need a decent 12v source to feed the controller/brake circuit & a proper socket for the trailer plug. I would suggest an electrical source connection more robust than a cigarette light plug. I put my harness together with some 10 awg THHN wire I had laying around for the connections to the socket & for the battery connection, using some jumper cable clamps so it is portable. I also wired in the 12v supply pin on the socket so the dumper battery gets a boost while connected.
I have not yet needed to use it since construction, but it is available if I do (& it will be with the 3038e - not going to move any significant weight in the dumper with the 1025R again).
Nick