Silly (but possibly smart) question regarding 2.40:1

[4styler]

I'm running a Mitsubishi Hc5500 and I thought to get rid of the pale gray letterbox with 2.40 films and not fork out $2000 AUD for a cinemascope lens, has anyone tried setting up a 2.40:1 mask in front of the projector lens? (I tried with cardboard ) Obviously you can't put it right in front of the lens cos the blur on the edge shows up, but has anyone designed something like this? I guess projecting onto a creamish white wall doesn't help the pale grey 2.40 crop but I thought why spend so much to get a 2.40:1 picture?