I built my own. Epoxy/Fiberglass over a foam core.
Seven layers of 'glass, so it supports my 250 pounds of weight.
There is fiberglass between the "seatpost" and the frame tubes, and there is more fiberglass wrapped under the toptube and seatstays, like the shape of the letter "U". So it stays permanently attached.
I find it comfortable, that I can move back on the seat to have more leg extension. And I also have my feet flat on the ground when I stop for a red light.