There are for this types several ways to build it your own.
You can buy a sufficient big enough trailer and cut out the size of a little door yourself fro the back lid, or as the guy below did, build it all the way yourself.
There is also the choice (which is yours offcourse) of building the trailer yourself or buy it prefab, so you only have to build what gos on top of it.
Plenty of ways to do it, but do it the way that suites you the best and/or think is the best.
what counts here too, is that there has been used a standard bow tent, which most probably needs a little remaking to fit for the trailer.