How to cut metal sheet part from electrical lamp base?

[DIYpowered]

Hello,

I am a DIY'er and there are some wall/ceiling electric lamps to install but the base is completely flush with the surface so there is no groove for the electric wires to pass through from the surface trunking. I think these lamps were meant to have their wires go through the wall/ceiling and into the base. But in my situation, the lamps need to be installed on the surface instead. So, i want to ask how to cut a defined area piece of metal (without scratches or distortions) from the base and without any expensive equipments?

I have managed to cut one of the bases using a saw blade.


But for the other bases shown below, the same saw blade does not work and i think it's because of different metal. The metal part to remove is shaded with a pencil on the inside and outside of each base. I have tried to drill the shaded area using a metal drill bit 5mm HSS from the inside of the base but it keeps slipping.


Maybe i need other tools? I did some research and i am considering to buy these:


Any suggestions or ideas would be much appreciated. Thanks in advance!

 

[footprints]

Just use the edge of a square file.

 

[foxhole]

Screw it down to a scrap timber so it can’t move .

 

[Murdochcat]

A round file comes in really handy

 

[Burnerman]

The saw you show there is a modern day replacement for the Abrafile........anyone remember them?
That saw will work just fine, but the work needs to be firmly held - whatever system you choose.
John

 

[foxhole]

Would’ve been easier to put a disc of timber on the base to raise it and allow cable entry.

 

[thomp1983]

You can drill it as you originally planned but you need a centre punch to mark the centre first to stop the drill wandering.