I wouldn't be too concerned if the overflow is in shallower water. It'll be impossible to design an overflow screen that will keep fish from escaping and not plug up with debris just when you need it to be clean to let water out.

At least I don't know how to design one that'll be self cleaning.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).