Here is my favorite:
http://home.att.net/~rstockman/overflow.htm
I did not make holes in mine, but used my chop saw and cut 4 or 5 cuts, 1/3 the way on either side. I did not use glue but it did take some 2-3 pieces to get the height correct.
I like this design because it can be removed just by unscrewing the whole thing out of the bulk head, some Durso types are there for good. I did have to clean mine once in 8 months.
Lee