DIY external over flow

[Jason McK]

I have purchased a Oceanic 40G stretch Hex. To save on room in the tank I would like to build and external overflow.
My Idea would be to cut a 6 inch (or larger) rectangle out of the back of the tank. Then insert a piece of acyclic that has the overflow cut outs (doing such fine cuts in the glass scares me). Then build a box around the cutouts to house a Derso.
Any suggestions or comment would be great.

J