Unfortunately I have never had anyone to teach me how to do some simple things properly, file a straight line etc. So now I need advice on what tools to use and correct procedure on how to make a good job of cutting out the middle.
Cover the area you're going to mark with a fat tipped Sharpie or marker pen. Transcribe the pattern you wish to replicate with a scriber or craft knife. Drill holes at the inside corners to show where you're going to cut to. Using a large drill, remove some of the bulk from the middle then, using a sharp, good quality file, file up your Sharpie line. Finish with Emery paper then Scotchbrite.
Is it steel or aluminium? either way protect the meal from marks from a vice.
Cut outside size with a hacksaw and leave about 1 mm for filing off later.
For the inside use a pillar drill (if available) and space the holes the same diameter as the drill, again leave about 2 mm from the inside line.
File slowly and use rules and eye to close in on the correct shape.
Finish all edges by draw filing and 320, 600, 1000 emery and polish or paint to suit.
Big shout to Frank Catt, Paul Fleming and Bob from Can Am Cars Ltd. Christmas has come early this year.....
...shows some marking out for my clutch cover on stainless steel 16 SWG (1.626 mm) thick.... it was a real workout filing that thing to shape. Also a little further down more marking and filing to make the cover plates, again stainless. I used about £20 worth of silver solder putting the mesh on!
draw filing is the right picture, 'normal' filing on the left.