It's really easy to make a new overflow if you want. here's a pic from my first 20
All you have to do is figure out the size of the box you want, get a piece of acrlyic that's the same size as all three sides, and then cut some teeth. Once the teeth are cut, you bend the piece to the shape you want. And just silicone it in place.
Silicone indeed does work for this application. You're not building anything structural. You're just 'sticking' a piece of plastic to the glass. And as long as you have a good bead on both sides, it should hold. The only thing that could weaken it over time is repeatedly hitting it with a big chunk of LR. I don't know about you, but one or two bangs of rock against the side of the tank (you all know that sound) and I get really careful.
What happens if it leaks around the overflow box? as long as you have a standpipe, the tank will only drain down as far as the normal level in the overflow box. Sure, you might overflow the sump, but your tank will still be fine. and most people only keep the level in the overflow box a few inches below the tank level. SO figure out the gallonage in 1" of tank and account for it in the sump. on a 36x18 tank it's 2.8 gallons per inch of tank height. that's easy to cover. And it won't ever really 'pop off' the tank. ithe hole/leak will start somewhere and slowly get bigger. And who here doesn't look at their tank at least every couple of days. You'll see the water level go down in the main tank and the water rise in the sump and know something's up.
If I were you Kris, I'd get a non-drilled tank and drill it where you want. Why pay to un-do someone else's work?
I'll be routing teeth in acrylic sometime in july or august. I'll post pics.
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