How about a carbon fibre plate?, you could get your plastic one used as a former.
I know they're not as stong as ali, but my pro-carbon one on my KTM exc-F has taken a big hit from a hidden rock during a race & got off with a reasonable scuff (DNF'd due the flat front though!!).
But they are smooth in the lines and fit really close to the frame and can be formed around casings where getting an ali fab could be difficult.
Might be worth a punt... don't think it fits the cheap criteria though... sorry.
Failing that, give someone like Patsy Quick a shout (Team Desert Rose), she'll know someone who can fab up a 'proper' rally gaurd.. again, probably not cheap.... but tricker than Paul Daniels whose just been out-tricked by Houdini.