In article .com,
" says...
i want to buy a circular saw - i have a few comp cases that i want to
mod now and in the future.
THey all seem very expensive. but this one is cheap.

http://diy.abcaz.co.uk/5_429772/powe...erm-18v-cordl=
ess-circ-saw.html

Judging by the URL that's a totally inappropriate tool (I didn't
follow the link).

I can get it for =A345 including postage.

I have a jigsaw but i think that messes up the case a bit. I might just
use a hacksaw. But I like the look of this thing.

Have a look at nibblers or aviation snips if you don't want to use a
jigsaw.

also, Is there any standard thing to cover ones mouth to protect
oneself from metal filings?

A neckerchief - it goes well with a wide-brimmed hat.