Not only is it possible to fit a mattress in the boot, but also to sleep in there.! My wife and I went to a music festival a couple of weeks ago at the other end of the country and didn't have anywhere to sleep. No problem.. I removed the rear seats, covered the floor in two cushions from our sun lounges, slid the front seats as forward as possible and made up the beds with pillows and blankets. So when the concerts was over, we snuggled up comfortably in the back and kept the OSS slightly open for fresh air. We sleept so well that we actually overslept the next morning..
Amazingly versatile our little A2's..!