Hi PD.
Along time ago, before autojumbles, swap meets, online choices etc a fella by the name of Robert Drury started a kitchen table business producing the simple bits that had rusted away...chainguards a speciality. I bought two, back in the mid 1980's. His technique was to make a symmetrical sheet metal blank, folded down the mid line to form the top straight of the guard. The flair at the back was formed by folding the oversized rear edges towards each other, overlapping them and spot welding together. Rough edges ground off, supplied unfinished, in the raw, they were of very good quality, and great value. Strictly low tech, backyard engineering.
He appears to have been very successful, "Robert Drury Quality Products" is now trading as "Robert Drury Precision Engineering" in business in a big way with a very specialised product portfolio. No cheapo chainguards now.
So, adapt something to fit in the short term, keep looking in the usual places. Buying a new pattern guard from India carries the usual assurance on a quality first time fit. Or have a go at making your own.
If you go down the homespun route, cardboard templates are the way, only cut the metal when you are happy with the template.
Look away now if easily offended....Newish old 40 gallon oil drums (a very reliable source of sheet steel) and scrap washing machines are the sort of material you can play with. Cuts with a well oiled jigsaw, even easier with a plasma cutter.
Swarfy.