I took some foam pipe insulation and duct-taped it around one of the handlebars of my deer cart for padding.  I then used a ratchet strap to lash it (I used a clove hitch, if you are a knots person) to the back frame of my bike.

It's definitely not as good as some of the connections I've seen from more talented individuals with fabrication skills, but it works really well in pulling the cart in mostly straight lines across long distances (think logging roads and such).