In the late 70's, my underseat tail light mount was made from a bit of Dexion shelving and a separator plate
Experience told me I could do a bit better this time around and I tried all sorts of rear lights before I settled on the Aprilia version.
I had a std square Z650 lamp with red and then with white lenses on the std mudguard mount, then I had it tucked under the seat fairing, then on a Z Power type extendable Z900 mount/fender eliminator.
A round KH/Z900 style, again with red and then with white lenses on the same Z Power mount came next......
.... then a GPZ600R tail light on a home made mount, then twin square and then twin round custom type rear lights on chrome mounts.
All were OK but never quite hit the spot for me.
.....but then I got talking to a clever French guy online who had used the 'prila setup that I have on now. He created the mounting bracket schematics I used to get a stainless steel mounting cut and formed, then polished and its been on there since.
If I were the entrepreneur type, I would shove a few into production ( although I would use ali not stainless next time)
but I am not, I am too comfortable I s'pose - that's the problem and likely why I am not rich