Mounting for a rack mounted rear light on a bike with no rack

Very much a plumbing solution for fastening pipes/tubes to stuff.

Larger diameters tend to look less agricultural - clamp screws are smaller in proportion to the rest of it - and the cast ring is usually more substantial than a P clip.

I'd be thinking about filing the end of the stud to the same angle as the seatpost then silver soldering a cross piece / mounting plate to offer a vertical surface for the light.
I'd also use a locking nut rather than locktite, and possibly replace the allthread by counter-bored 10mm bar threaded at one end only. Eventually it evolves into a seatpost-mounted-saddlebag-support-with-fitting-points-for-a-light, but that's a clumsy name so let's call it a rack for short?