Rebekah~

I think someone else mentioned this but have you thought of using a king-sized pillow case? We actually had a similar situation....my parents gave us the bassinet they used with me. My mom sewed an adorable eyelet cover for the outside and everything was in great shape except for the mattress. We bought a super firm king-sized pillow and bought a few pillowcases for the cover. I washed them in hot water so they shrunk a bit, and they fit real snug. Still waiting for baby, so we haven't actually tried it out yet, but it seems like it will work great.

HTH
Jera