Spacer tubes mate, spacers cover all threads and look cool and cover most situations.
My first encounter with the concept of 'spacers' was aged 4 ....a portable BBQ my old man had that was made of 2 plough discs with sets of three steel rods welded on each disc at 120 degree intervals, one set under the top plate disc and one set over the bottom fire disc with another set underneath.
Three long bent pipes and 3 short ones (
spacers).
All you need now is the measurement. But as you measured stuff to design your offset QD ones to match the stock sissy bar you have already have a good idea, he has flat brackets and you have 4, 5 or 6 Rockets (at last count) so you should be able to figure it out.