L200 and L128 both reach that size, they don't rasp driftwood much, but will use it for cover quite readily. Most plecs that rasp driftwood are going to also hit the 18" mark or more. All of the Leporacanthicus species would be good, also L66 L333 would be good. Depends greatly on what your preferences are. Narrow it down and we can help point you in the right direction.