Rusted Air Box repair/replacement

[Jacobm99]

I took the driver's side fender off my 72 blazer to replace the lower A-pillar and noticed that the air box was rusted out (I think this is what it's called. It's the section where the debris that enters the cowl ends up).

1) Does anyone sell a remanufactured replacement (can't find anything in LMC or car & truck shop)?

2) If there is no replacement piece, is there anything wrong with fabbing up some sheetmetal and replacing it?

3) Why can't I just cut the bottom section off (I don't really see what purpose it serves; structural support maybe?)

4) Any suggestions on preveinting this from rusting going forward (I think I remember seeing a thread with a screen being mounted under the cowl- link? anything else?)?

Thanks in advance.