Add a bunk

From my experience the upper bunk was always hot in the summer and darn colder in the winter so as far as I'm concerned the upper bunk is mostly worthless but if you must do something make sure that you you know that the upper bunk will Shake rock & roll during driving times and even the factory built upper bunk in our Freight Shaker is loose and full of cracks we don't trust it so we use it for storage

 

I'd check with a salvage yard and see if you could find a factory set up, or a dealer, but I'm sure that would be spendy.

 

No idea if this would work, but how about a hammock. Check with places like Rei and online like Gossamer Gear (ultralightweight camping). Some hammocks are more comfortable than mattresses. No idea how to support it tho, reinforcing or a frame perhaps.
Pretty truck. I start school Monday, have never been inside a big truck since I got to drive a cabover around Mid Ohio track sometime in the early 70's.
Take care,
Mary

 

Looking at just your pics I would say go with a fixed bunk built on a aluminum frame. It appears from the pics you have the room between the tuck and roll and that top set of storage. Or if you could find a top bunk out of a Pete with the back half fixed and the front part folds up.