Drilling/tapping in ends/side of aluminium plate

[EddyCurrent]

You could clamp a piece of wood on either side, two pieces of 75mm square for example. If done correctly this would create a flat area with the plate end down the centre at 90 degrees to the surface.
Then you could attach something like this; http://www.ebay.co.uk/itm/Press-Benc...item566de7aea9 to the wood so it's central on the plate and drill your holes.