Hmm... I've never tried any challenges on here. What an interesting one. I don't know if this is what you want, but this piece contains 5 notes [B,C#,D,E,G] and it has 15 measures. There are 5 midi instruments.
5 is the magic number. This took me 5 minutes to make. Though I can't say it's marked at 5 bpm.
5notes.mid