As a tightwad, I resisted buying underquilts for a long time. I used double foam pads in my single layer hammock at temps in the low 20* F. I rolled the pads and hung them off the back of my pack with small bungie cords, for the Everready battery look.
With 24" wide, double pads, you can offset them slightly side to side to give you a double layer under you, but only a single layer at each elbow. Some carry one full length pad with a half length to use crosswise for shoulder and elbow insulation.
Note that for serious cold, those hangers supplement all of their other high dollar insulation with pads. Very cold air is dry so you get less condensation on the pad under you at 20* than at 40*, it is your breath that gives you the frost trouble at 20*.
thru-hiker.com sells Climashield(CS) insulation. Read about the KickAss Quilts made with CS at arrowhead-equipment.com to see the possibilities of using synthetic instead of down insulation in a DIY underquilt. You could combine Insultex from backwoodsdaydreamer.com (BWDD) with CS in a nylon envelope following the BWDD tutorial for Insultex underquilts.