does anyone make a rear slide butt plate for .45 shield or do they all interchange between 9mm/.40/.45?

Yes they make them for the 45 and no they don't interchange with the 9/40.

NDZ Performance makes them for the .45, I put one on mine as I don't care for the plastic rear slide covers. Not that the plastic ones cause problems, I just prefer metal.
I ordered my NDZ rear slide cover in FDE, as I was planning on Cerakoting my Shield soon, it saved me the trouble of coating the small part. (Hard to hang it in the oven without a hole for the wire).

NDZ also makes mag base pads with a finger extension. I use that for my carry mag, the other two mags in my belt pouch are +1 (7 round).

__________________
U. S. Army Veteran