I had a metal shop make me some out of 3/16 th steal. They will rust but they are wider than the big box ones (7x7), so where you bolt them to the corners you are farther from the edge.

61 acre water shed lake. bass, channel cat, black crappie, wiper, walleye, redear sunfish, blue catfish and bluegill. To many bullhead and common carp
http://www.pondboss.com/news/videos/why-subscribe-to-pond-boss