Yeah they'd be pretty easy to make up - just some thicker Lexan from a bodyshell off cut or whatever cut so you could bolt it behind the shocks and then folded up over the top or something - sure people could quickly make something anyway, even from an old rear wing
I do think it'd work though, the main benefit for me would be all weather racing as the water shouldn't gather anywhere if it's done right, and grip would still be pretty good with the right tyres....
PLUS the Tyres wouldn't wear out!! Not for ages anyway.... and what do people moan about most??
Anyway the other thing with breaking stuff, is you really would have the same problems racing on hard compacted dirt anyway as in the baking sun those tracks are as hard as concrete anyway!