I installed mine on a Oceanic tank. I played around with it a little to figure out the height and ended up flipping the template upside down and putting the bottom (thinnest) against the tank trim. I clamped it in place by using a block of wood on the interior of the tank and a c-clamp.
I can't see the waterline, but because the Oceanic rim is so thick I put a spacer against the exterior glass to hold the box away rather than attempting to cut the plastic. You could even use the template as the spacer; I realized this after making one out of 1/4" azek.
Silicone the spacer to the glass and then have this arrangement: flange, gasket, glass, sealant, spacer, gasket, box, nut.
I also put an 90 elbow in the box on the main drain line because I was getting some whirlpooling. That and extended the emergency standpipe to reduce trickling sound. I'm running what I believe to be 4-500 gph through the 800 box and I wouldn't call it "dead-silent" but it's barely noticeable; slight bubbling water sound.
It's well built. I need to contact them though because the removable weir has a gasket in it that if I push the weir all the way in it will leak past when the return pump is off and lower the water level below the weir teeth... that is it will drop to the bottom of the drilled hole and that floods my sump. I have the weir box out slightly and that seals the situation up. Hard to explain but it's a bit of a flaw. I *think* an additional o-ring will cure it.