I built a riser using five-gallon buckets. it was 15.5 inches tall.
Here's a link to another thread where someone built a riser using 3 1/2 ballon buckets. they might do the trick.
Actually, for me the 5-gallon buckets were less expensive than the 3.5-gallon buckets. The 15=inch height brings my head up to the level of the rest of the band.