Making a cutout in an Oceanic overflow box

[jarhead]

I would like to make an additional cutout in an Oceanic's overflow box so that a loc-line cut rest flat on top of the box. Any idea what I can use to make the cut? Will a Dremel tool do the trick?
The pictured tank is not mine - I am just using the idea. Should I just get rid off the third (leftmost in the picture) return output and settle for two. This way there is no cutting required.
I have marked in red exactly what I am trying to accomplish:

 

[Anonymous]

hi.
Dremel will work. If you want a semi-circular cutout, clamp a piece of wood behind the overflow, and use a hole saw.