B&Q sell small sheets of both steel and aluminium - at a price. If you
have a metal fabricator nearby, ask them for offcuts. They can be found on
most industrial estates. Small amounts they'll often just give you, or
sell larger amounts at near the scrap price. A pukka supplier is likely to
only sell large sheets.

Cutting thin sheet with a jigsaw requires it to be well supported either
side of the cut. The more usual way is with tinsnips, which are also more
accurate.

How neat will a job like this done with tin-snips be? And how tired will my
hands get. I need to weigh that against the cost of gettings somebody to do
it for me. Its only going to be a piece about 60cm wide (24 inches), but
about 20 cm high (8 inches) with rounded corners. Something like this :

http://www.chowns.net/cab/panelshape.jpg

Sanded smooth and bolted down to another piece of metal, and then painted.

Regards
Stephen