For alloy mudguards, I wonder if the easiest way would be to purchase a 5" x 19" motorcycle mudguard like this https://www.britishbikebits.com/alloy-universal-rear-mudguard-5-wide-c-section#.WqqAIi90fy9 and then get someone to TIG weld on the flat mounting plate plus a couple of bracing alloy strips up the flat plate and onto the inside of the guards. Easier and cheaper than having fully custom made ones.

Wilson