Now I only very recently fitted a new tailpiece puzzled together from leftover XS400 parts to my everyday TR1 and honestly I wasn't quite happy about it. One of the key elements that bugged me was that it was almost as flimsy as the old SR500 rear mudguard I had used before.
Quite a while ago, I had bought a 160mm zincplated trailer mudguard with two bevelled edges (copy cats beware, there's some which are meant to be fixed directly to the sidewall of your trailer and thus have one FLAT side. They are obviously no use on a bike unless you really want to go all funky.)
So what I did was to measure the width inside the rear subframe, split that in half and come up with two aluminium bushings to center the mudguard in the frame. (Only to realise the error of my ways and then notch them on the mill, because the holes were drill to low and they touched the lip of the mudguard...)
On the right you can see the finished product. I haven't employed some additional holding strap to the front of the mudguard yet, because the notched bushings actually hold it in place with ease.
(But I have to get some thin stainless material to make this look and work properly!)