I just put the 12 inch light in my biocube 16. I did it kind of the easiest way which was just to using the included plastic brackets and have that sit on the inner lip of the biocubes top rim. I did have to remove the splash guard on the lid to do this, and ended up uninstalling all of the rest of the hardware as well just because I wouldn't need it anymore. It definitely could also be installed in place of the existing lights and still install the splash guard over the Blade, but you'll have to find a solid way to mount the light there so it doesn't move around. definitely possible. As for spread, I am quite happy with it and corals would be able to grow throughout the tank, but it does visibly fall off at the last inch or two on both the front and back of the tank. Maybe at some point I'll get a second one to even things, but it works and looks good even now.
Totally thinking about doing this in my BioCube 32. Here is a video I found of a 12" in a BioCube 32. They claim to be getting 120 par on the bottom of the tank in the comments.