You can get overflows in just about any size you want, granted one of the ones mentioned above is 24" however not sure if you have 24" inside the tank, you could always do some surgery on the box... but I totally get not wanting to have to modify something brand new Out of curiosity is this an acrylic tank or a glass tank? If an acrylic tank I wouldn't even bother with a Shadow overflow, simply get an inside box and outside box, and you can get some holes done and fuse it to the tank itself. If glass, then yeah get a box, you can do glass overflows but those can be tricky to make yourself. Also I ask because putting something on the bottom sounds like something you do with glass tanks, and not so much acrylic. And yeah you can do that easy enough, Tap will cut to fit but they're pricy for what they offer. You can get smaller sheets cheaper through Amazon or someplace and simply cut to fit. As for the reinforce seams, they basically put a triangle shaped wedge in each corner (I think it's acrylic, might be glass though) and they goop it up more. I wouldn't worry about doing this to a tank though, custom tank builders like to overbuild tanks so that they have no chance of failing, not to say that an off the shelf tank will fail. But glass will fail eventually as the silicone won't hold forever.