Get some rubber sheet, cut it to the size/shape you want, drill a couple of holes in your mudguard and the rubber and use some small nuts and bolts to attach it, don't tighten too much though.
I have used this in 3mm thickness successfully but I have also got some 2mm, which I haven't used yet.
Handily RAW mudflaps also publishes the size of all their various flaps which you could copy. Or of course you could just buy one of their mudflaps but that's not as fun.
Get some rubber sheet, cut it to the size/shape you want, drill a couple of holes in your mudguard and the rubber and use some small nuts and bolts to attach it, don't tighten too much though.
I have used this in 3mm thickness successfully but I have also got some 2mm, which I haven't used yet.
Handily RAW mudflaps also publishes the size of all their various flaps which you could copy. Or of course you could just buy one of their mudflaps but that's not as fun.