halfords or similar , sell a plastic moulded radio tray. fix it under dash and put a support strap at the back. Have two fitted, cheap and effective.
this sort of thing....
http://www.ebay.co.uk/itm/Seat-Ibiza-Mk-3-02-09-Black-Underdash-Tray-Single-DIN-Car-Radio-Fitting-/131125166214?hash=item1e87ab4086:g:zKYAAOSwD0lUdPSY