The light head was originally cut with an electric hacksaw. I was going to dremel out the hole but got a bloke at work to do it instead on the mill. He was kind enough to tidy the ends too. You could do this easily enough with a hand saw, will be a bit of work with a dremel making the hole (thinner al would help, this stuff is quite sturdy).
The bracket was cut with a band saw and cleaned up with a file.

Just been out for a pedal, ride report and photos here.