Since the acqusition of special Boards or Mirrors or such can be problematical in Europe, and because you said "Cheap", I'd advise that you paint a screen on a smoothed wall surface. Can this be accomodated?
If so, applying a Gloss White Paint first, then a Overcoat of a Flat White Latex Paint mixed with either a few drops of Lamp Black, or better still, the approriate amount of of a Silver Metallic water based Paint (aka or similar to: MMUd-SE ) will deliver you an excelent image with plenty of Brightness, work to keep your CRT Blacks as black as they should be, and wind up costing very little overall.
Even if you need assistance in procuring the special Silver Metallic paint, the additional cost of shipping form the states will still not send the budget into a tailspin.
PM or email me if you need specific advice at length and I'll send you links to the info you need / request.