I got brave enough with the materials to try using a space blanket and polyester fill sandwich hung as a conventional UQ. I have posted a couple times on using it inside an under cover.
* I took an AMK double HeatSheet space blanket, folded it over to make an envelope and stuck the sides together with double-stick tape.
* I put a 45"x60" (crib size) batt of Poly-fil brand Extra-Loft polyester insulation in the envelope and sealed the remaining edge with self-stick Velcro dots-- this is a prototype and I wanted access to the inside.
* I tied lines to the corners with a lark's head, capturing a hank of the insulation at the same time.
*The lines run though a closed loop of shock cord.
* I ran the lines so they are on top of the hammock ends. This helps to lift them and keep them and pull the UQ close to the hammock body. The tension can be adjusted by making an overhand loop in the lines, or use a toggle. It could be hung from the whoopie slings with a snap hook hung on Prusik hitches and adjusted that way.
I tested it on a Grand Trunk Ultralight hammock with a modified suspension. The width is perfect and the length is about 60", so at least a 2/3 UQ. It is warm and I think good to 40F or so with a decent sleeping bag and pillow to make up the length. A taller person may like a foam foot pad. I'm 5' 10" and 220lbs.
Weight is 11 ounces and loft is 1/2". Cost was $7 for filler and $5 for the space blanket. I had line, shock cord and tape. You can certainly pull it off for under $20. Assembly time is under an hour, including hanging it. It will stow to a bit bigger than a Nalgene, rolling it up carefully and securing it with a rubber band.
It could be sealed with duct tape, which would reinforce the corners and toughen it a bit. That would add a few ounces and make it harder to roll up. I would leave some "bleed holes" to let air in and out and dry any moisture captured inside. I think duct tape on the corners would help. The filler should be stabilized somehow-- using tape, glue, or stitching through with tape reinforcement patches and maybe buttons.
After doing some testing, it works better to use the shock cord loops in a Prusik around the suspension lines. A higher attachment point helps the shape a bit. The are some problems with sagging at the ends, particularly at the foot end-- my shoulder fill out the top better. Shape can be adjusted by tying to the fabric closer to the center and folding in the excess. Doing that does shorten the effective length. Another way would be to just gather the slack and whip it lightly, or roll it up. You could gather and whip the ends of the sandwich, creating a asymmetric boat shape.
If you really wanted to make a weatherproof setup with these materials, you need two space blankets to get the size. Then you could whip the ends like a hammock and create a complete under cover. The insulation would necessarily need to go the full length; it could be taped to one layer. I wonder which layer would be best.