When I was working at Quickco Motorsports we developed a rear brake kit but very crude as not much was available..
Nowadays the best cheapest option is to use focus mk1 (LR) rear rotors, calipers and possibly handbrake cable.
The caliper adapter plate will mount on the inside of the 4 bolt flange, diff centre side.
As for spacing the squarey plate, we used to use just washers which took up the thickness of the backing plate of about 2mm . If your backing plates are stuffed, you could cut them back to the squarey plate and that's that part. Thickness of rotor hub is about 6mm which is the measurement your after, nothing else changes. Mount caliper at the back of the axle as focus has, plate thickness can be as thin as 6mm but wouldn't go higher than 8mm as it's only the rears , this bracket can be mounted by just 2 of the flange bolts, if you want to be sneaky a secluded weld stitch or two in hidden zones for piece of mind.
I did my rear calipers way back in the 90s and just welded the brackets on, never a problem tho later when I bent a housing I decided to bolt and weld the brackets on. Calipers I used back then were Telstar turbo and rotors were Volvo 264 machined down to 245mm then redrilled the stud holes for escort .