I wandered in late.
Other thoughts.... you could use spreader bars wider than the truck bed but not suspension triangles etc, if you can rest the spreader bars on top of the truckbox and keep them from moving in. You could build some kind of frame that hugged the edge of the truck box and keep the spreaders in place.
But TeeDee predictably makes a really good point. 20" is very shallow to be getting in and out of a hammock! As for depth, it will be tight but probably can be done. The height of my bridge hammocks from base to spreader bar is 16-18 inches. My butt no doubt deepens that some, but the height just depends on the width of the fabric between the points of attachment on the spreader bar, and you can easily make that 16-18 inches into 12-14 inches.
fun project! I wonder if I can rig a hammock inside my Outback? ...
That is so cool!
The present moment is eternal. I would rather be Here, Now.
I had a cot that I used for weekend campouts when I was in the National Guard about 40 years ago. That cot is still in the basement I think. It had a frame that was made of aluminum tubing with four "U" shaped legs. With tubing as side frames. There was not cross frame other than the legs. Fabric for the cot was very loose from side to side. It would lay very much like a bridge hammock. I would be only 2 or 3 inched off of the ground with this cot. If you could fabricate some type of frame something like that could work.
Another idea is Grizz's four post vintage hammock suspension for an in truck bridge.
No suggestions that haven't already been given about setting up a hammock, though.
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