I needed a carrier for my enduro bike and i didn't want to have 
to pay the registration fee that a trailer would require. Also, 
storage becomes a problem with a trailer. When I dont use the 
carrier the bike simply sit on it in the garage. I started the 
project with the channel in which the bike rests upon. This was 
made with some angle steel I had laying around. I purchased a 
.250" 2" x 2" square tubing for the receiver bar. I reinforced 
it with some 1" x 1" (.062") square tubing. I also made two small 
stabilyzer receivers that I welded on to the bumper. I painted it 
flat black. My total cost of materials was about $50. (I already 
had some of the supplies)

FYI: In NY State, if the carrier does not extend 30" past the 
vehicle, there are no registration requirements.(at least that's 
what i was told)

Enjoy,
Walt
refer to pics:
dirtbike_carrier_1a (the bumper, i found plastic plugs for receivers)
dirtbike_carrier_1b (the carrier on the '93 jeep wrangler)
dirtbike_carrier_2  (the '72 yamaha ct2 on the carrier)
dirtbike_carrier_3  (side view of the bike on the carrier)
dirtbike_carrier_4  (closer view from the side)