I built one a few years ago, its 72"L x 26"W and 10"D. I got some thin rectangular tubing from work, it was too thin to weld so I had to pop rivet it all together then I covered it with 1/4" puck board. I built a hitch with 2" angle iron, I attached it where the front curve of the toboggan meets the flat of the bottom, this way when I'm on a bush trail I don't have to worry about hitting trees with the leading edge of the toboggan, also if I go over a tree the stronger hitch rides over and not the front of the toboggan.
After my first trip with the boggan I found some poly strips to use as runners so it didn't whip back and forth when I ride on smoother surfaces.
It has served me well for the last five seasons, I even use it to haul wood out of the bush in winter, so it has seen some heavy and hard use. But I didn't paint the metal so now its getting pretty thin.
I want a bigger toboggan now, and one change I would make if I built another one is I would like there to be skis under the toboggan bottom, I like the toboggan for the flotation in deep powder snow, but in deep slush it just all piles up in the front and stops the snowmobile, so with some skids under the toboggan it would let the slush slip under and hopefully let me get enough speed to plane up and get on top of the snow.