Forget buying plans, it's fairly easy --  I'll try to explain it here, but a better source of diagrams might be found in your local library, if they have a collection of old books on stage magic and illusion, as mirrors are the basis of a lot of the tricks used in very old stage illusions.  The cart _must_ be symettrical, and the height of the cart should equal the depth.  Run a reflective plexiglass (Don't use a real mirror, you'll slice someone to ribbons if it breaks.) sheet from the bottom rear to the top front.  paint a pattern on the interior bottom to be reflected to the "rear" of the cart.  Experiment with dressing the sides and rear of the cart.
 
In the rear of the cart, behind the mirror is a ledge that your actor can sit/squat on.

Here's a diagram, most definitely not to scale, and I think my hand-drawn "reflection" is slightly off, but it's enough to give you the gist of it:  (And ignore the half-person, his lower half just wouldn't fit. :) )



It's going to look pretty convincing straight on, the more you angle to the sides, the more you're going to lose the illusion.  Experiment with other mirror placements, too, to see what works best for your sightlines.
 
Good luck!