We no longer have a hybrid, but when we had it and our popup, we used foam mattresses, cut to fit the area and encased in a couple of flat sheets.  We got rid of the mattresses since they were NASTY when we purchased.
I used an eggcrate pad on top of the mattress, then sheets, both top and bottom and an electric blanket with a quilt covering it all.  We were toasty warm sometimes, too toasty.  
We could camp down into the high 20s with this setup and we also had a small electric heater to take the chill fof.