Free: Contests & Raffles.
I use a snowmobile deck myself. I wouldn't put a frame across your bed rails as it'll break down your bed sides over time with two quads up there bouncing around. It would be better on your truck to have the weight down inside the bed, also with an ATV deck I'd have it side load instead of rear load like a typical snowmobile deck. Is your truck jacked up 6 inches on big tires? If so you'll want a telescoping ramp or just winch the quads up.
If it's not too late you may want to add a sliding "floor" on the bottom. That way you can load and reach supplies all the way to the front of your bed.
Just out of curiosity why go this route instead of a trailer?
I used a deck until the kids started riding dirt bikes. We now have a toy hauler. You can get a lot nicer TT for the money compared to a TH. I love the TH for the ease of hauling the toys, one King Quad and three dirt bikes, but miss the comforts of the TT.
That does look good, can you hang the ramps on the sides so you can drive on drive off? Those ramps will get pretty skinny backing it down off the deck.