The 924S steel backing plate is the ticket, but they are extremely hard to find and expensive. The problem that I had with the 944 aluminum plate is that it would not snug up, thus leaving that 1.5mm gap. Yes, a spacer would do the trick, but the thing that made me nervous was that the stock swing axle bearing cap and washer assembly with the weep hole had to be discarded. Discarding this washer and bearing cap could cause transmission oil to leak into the drum. The aluminum 944 plate has a different sealing mechanism because it is only holding in the grease which lubricates the stub axles rather than the transmission oil you would find in a swingaxle. Oil on the drum could foul the parking mechanism or worse the pads. Not something I want to happen in a hard braking situation. Obviously the hotter and harder you run the trans, (not to mention the degree of camber) the more viscous the trans oil becomes and the greater the chances are of leakage. Regardless, this is a chance I was not comfortable taking.
I think that MBT just did a production run of their version of this backing plate, but you would have to contact them. Hope this information helps.
www.mbt-engineering.de