I used a hole saw and it butchered the alloy and made terrible mess of it.
Also intrigued, as the one I made had a load of channel sections under the plate to act as structural supports... which I then realised it didn't need most of!
Do you have anything under or just the axle mount supports?
Also, what thickness plate did you use, what process did you use for folding it (never thought of that meself!), and what material did you use for the axle supports?
Step drills work well
You can get them from most engineering suppliers or ebay - Maplin do a set for £17
http://www.maplin.co.uk/3-piece-titanium-step-drill-set-354167