Richard; Here's the pics of my tailgate.
The pipe hinge didn't come out in the pics. It's two pipes one inside the other. Take the outer pipe and chop it into segments about 8 to 10" long. With the two pipes assembled weld every other segment of the outer pipe to the dovetail section, and the opposite segments to the end sections.
If you have to buy the pipe get black pipe for the outside pipe so you don't have to grind off the galvanizing to weld it.
For smaller diameter pipes for a thinner tailgate the inside "hinge pin" would probably be a steel rod.
The tailgate doesn't have to be cut in the middle, you can have the end section only a foot or two long leaving even less sticking up.
You need little legs on the end section(s) to support the joint when you drive something up on it.
My tailgate has a coil spring assist to help lift the end sections. A leaf spring like they use on horse trailer tailgates would have been better.
You probably wouldn't need a spring assist. My ramps are made of 2" x 3" schedule 40 box beam. A guy drove a fork lift up them the other day to load a skid of fertilizer. They're really heavy ramps. After 10 years the chains pulled the corners out of the end of my trailer. So I had to weld some scabs on the corners to reinforce it. That's the only problems I've had.