I'm not sure how the Biocubes are setup, but if you're looking to set up a sump with yours, would you be able to drill into the tank? Drilling is better when possible.
If that overflow box won't fit, you can easily make your own. There's a lot of literature out there on how to do it and it's pretty simple if you have a dremel and some time to kill.
Making one wouldn't be an issue. I'm currently in school and we work with acrylics and such. But if the Eshopps box fit I would have just used that one instead of trying to make my own.