You don't even need to mock up, all the dimensions are known, just need a 8mm plate each side to suit the caliper spacing(bolts), then another pair 13.5mm further out, and two 8mm spacers.
If not sure draw out on paper easy to do, I'd just use regular mild steel for this, DON'T use stainless steel!!
Sure saves a huge amount of money this way, but discs are defo 24mm, can find you some 22mm in the book of words most likely if wanted ?
tabetha