AlexKnoll - you 'da man. Love your setup and I'm sure we'd all drool to see the entire rig...more photos please!
I've been on the fence about trailer length and how to partion it. For now, limiting size to either, a/ a 7x14+ 2' V to stealthily customize or, b/ a gooseneck 2-horse trailer with DR to retrofit is the proportionate-sized ticket. Either way, its to satisfy an immediate need, a 'round 1' proof of concept, if you will, not likely the end game. I take it your M3W doesn't move side-side much in there.
Marmota - the trailer in the link I sent isn't mine. It's an old post. I included it purely for illustrative purposes to show the e-tracks. Personally, I'd never chose a single axle.