So ... you are 5'9" (I'm assuming you don't have prosthetic legs) and want to sleep in your 2dr wrangler lying flat on your back.
Lack of storage and 5'10" horizontal planes are your restrictions.
- you don't want to sleep in a roof-top tent
- you don't want to sleep outside the jeep
- you don't want to change the front seats to lie-flat
You obviously have to go up.
You then have two inexpensive choices and one costly but convenient choice:
- Sleep on a support structure supported from below. (ie a wood/metal frame deck)
- Sleep suspended on a hammock or cot that hangs from above.
Seems like material from a hammock or a cot material platform would be the most logical low-cost solution.
A frame structure takes up a ton of space and good luck with all that extra s*&t flying around in a wreck.
If you like the suspension idea
then you need to decide between hanging or tensioned material.
Hanging a "hammock" style bed allows for near-instant setup and collapses down to next to nothing but you and the little lady will be in 2 different sacks.
Here's a purpose-made off the shelf jeep hammock for $189.
Or, hanging an actual cot would mean just cutting the legs off a cot below the tension knuckle and tying it to the roll bars at the desired height.
Takes up some addl room but seems pretty quick as a temporary sleeping arrangement.
A tensioned material platform allows you to maximize the sq ft used and spooning with the missus on cold nights.
It also takes up nearly the same minimal space, offers the most sq ft of flat space and stows away quickly.
You could try 1" poles along the sides as stays or have a cable with corner loops sewn in along the perimeter using a come-along type ratchet.
I would suggest this over a hammock style setup.
Here's another off the shelf option that you'd just have to add solid mesh on top.
IMHO an http://theroadchoseme.com/ursa-minor-j30-pop-up-camper
Ursa Minor (hardtop with a built-in pop up top) would be the way to go, but that's just me.
If you do have prosthetic legs, ignore everything I said and use an inflatable kiddie pool instead.