We got a king sized 8" memory foam mattress from Wal-Mart. I used the top board from one of the twin beds as a template to cut the three corner mattresses for the back corners. To cut the foam I used a very very sharp hunting knife, a long fish fileting knife would well also.
You did not mention what bed you have. If it is a queen, you could get a mattress from Wal-Mart, open the end of the cover, cut off as much foam as you do not need and stitch the cover up, then put that end against the wall under the window.