you don't drill the lower, you drill the mid section, to do it right, you remove the powerhead, exhaust adapter, and the exhaust can. leave the can out, drill 6 1/2 holes from the top thru the bottom of the mid, exiting just above teh anti ventilation plate. Search on screamandfly.com for a detail write up. Heres a picture from the top
https://www.google.com/search?q=merc...d1GlKjM:&vet=1