Whatever metal-working skills I acquired during my training as
a radar fitter in the Royal Air Force so many year ago have
evaporated it would seem (RAF Locking 1958). Just now I tried to
cut some short strips from a 3mm aluminium sheet using a hacksaw
and made the worst possible mess of it.

No matter how I tried, I couldn't stop the cut veering to the
right. In the end I just cut out the pieces larger than required
and filed them to shape. The end result was OK but what a waste
of material.
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Joe , May I suggest you consider the following method :

--Take 2 pieces of angle iron ( say 75 x 75 mm , 3x3 inches ) ,or larger
with a length of at least the length of the cut to be made.

-- Place the 2 lengths flange to flange and drill 2 or more holes
(depending on overall length) and bolt them together with washers in between
; the total washer(s) thickness being a fraction more than that of the hack
saw blade thickness .

-- Place the bolted flanges in a vice ,such that the other (spaced )flanges
form a horizontal platform .

--- Clamp the sheet metal to be cut to the horizontal flanges and make your
cut, using the opening as a guide

Note : With a normal U-frame hack saw you would cut the washers and bolts
(depending on the blade operating angle), unless to move the clamped sheet
metal .
However it is better to use a knife type holder for the hack saw blade , a
low cost tool available from most DIY stores.
With this tool ,which only holds one end of the blade you avoid hitting the
washers (and bolts) while making a straight cut

Good Luck

Frank GM0CSZ / KN6WH