Rust? Where was there rust?
How long would you say the piece of acrylic is? The piece I just put on the
Aquaclear 50 extends the lip down about 6 inches, so the end will be in the water. I've rounded the pointed edges to prevent any eyes from accidently being poked out.
I also recently got an
Aquaclear 70 (I've gone kind of crazy with them

). Maybe I'll try your solution on it and if I don't like it switch back to siliconing the filter lip.
BTW, in your second pic it looks like there's a second lip at the very bottom of the plexiglass.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-