I bought these £25 5mm alloy spacers and just went out and removed my brake disc to get the spacers machined to fit.
I thought i'd try the spacer in the back of disc and to my surprise it fits like a glove no play but it needs a nats whisker of a taper on the outside to sit completely flat in the disc .
The stud holes are a perfect fit for the studs as i had removed
a stud to try and fits on the unthreaded shoulder great with only a whisker of play.
The centre bore needs opening up to the disc and the disc bore is the same size as the back lip of the flange. but you don't really have to machine the spacer bore out because the studs are a very good fit so you could just file it out.
So you could skim it with a grinder on the outside and file the inner without going to a machine shop or have the middle machined for abit more support.
I'm going for machining