[...] I'll do when I replace the bolts with four of the same size.
That's a very neat solution! One suggestion would be to replace one of the lower bolts with a quick release cam bolt, to give you tool-less adjustability of the light when mounting or dismounting a suitcase. A bicycle seat-tube bolt might be a bit large, but you may be able to find a smaller one...
something like this, perhaps?
https://www.kipp.com/gb/en/Products/Oper...less-steel.html
