Hmmm... we had a matress ina water bed frame until we moved here in Jan (we had a waterbed until i couldn't get out of it when I was pregnant hehehehehe).

Anyway, I jsut put the cot up to the side of the bed, take of the drop-down side, and any gaps i put my pregnancy pillow between the cot and the matress/bed, stuff it down so it was a really snug fit... Our largest gap was a few cms though, I don't think it would work with a HUGE gap. You could see if it levels up on the lowest base setting? I found if the cot/base was a little bit higher than our mattress worked a bit better.

Anyway, I'm sure they don't recommend it like that but it worked for us.