Iv been thinking of making a transit trailer for some time aswell. to match our minibus.
My thoughts were to get a swb semi-hi roof because our minibus is the semi-hi, remove the axle compeletely and put two trailer axles in so that it removes the nose being heavy. (this would mean quite a bit of work to panels and the wheel arch inside, but we know a guy thats good at panel beating.) then weld a bulkhead in securely to hold the front of the van sturdy once we chop off the A pillars. then chop it off in front of the bulkhead and fit an a frame to the front with coupling, and then make a fibre glass nose cone for the front to make the aerodynamics better.
I done a pretty shitty photoshop job of what it would look like with my previous thought of using a jumbo and chopping the roof down to a semi-hi here
http://fordtransit.org/forum/viewtopic.php?f=30&t=51675