I've found the higher priced models of the Intex line of air mattresses to be quite comfortable. Any air mattress will cool down with the surrounding air, so some insulating foam, blanket or mattress pad between you and the mattress would be good in cool weather.
Size that fits the Element is a tougher question. With the seats folded down, a double size mattress will fit. It will have to bridge the space between the front seats.
Sleeping on the floor with the rear seats up (or out) is trickier. With the front seats forward, you have a length of 6', more if the tailgate is down, but the width is about 42". A twin size mattress has nominal size of 39", so you will have to contend with a gap on one side or the other. Some doubles might fit the space if they aren't too rigid.
With the seats folded up a 5" thick mattress eats up the space between floor and seat bottom, which may reduce the effective sleeping space.
I've decided to go with a pair of tube type air mattresses. Pacific Outdoor Equipment makes such a mattress intended for backpackers. A pair of these or other 20" wide camping mats (e.g. Thermarests) fits nicely on the floor. REI is one place with a large selection of these mattresses. These have a thickness of up to 2", which is enough if inflated just right.
The other thing you have to contend with when sleeping in the Element is distributing the cargo that you are not using. A surprising lot can fit on the front seats, foot wells and dash.
paulj