Doug,
I rented a motorcyle trailer from U-Haul and the way it's set up is perfect. The ramp is made of expanded steel and the full width of the trailer. The beauty of this is that you can walk up the ramp with your bike and never worry about it feeling "tippy" as the center gravity rises above your body. The trailer itself is also very, very low. IIRC, not much more than 18-24" above the ground. The ramp is hinged (acts as a flow through tailgate) so you just pull 2 pins and drop it down.
They asked me how I liked it and I said "Where can I buy one?". :) They got a kick out of that. Anyway, I asked if I could drop by sometime to take pics and dimensions so I could build my own.....