I finally decided to install my modified 1223 Xray bumper and figured that I should take a photo of how I modified it to fit the spec r. The xray bumper is 18mm thick iirc, and the spec r bumper is 15mm. I milled a 1.5mm pocket on the top and bottom of the bumper to make it fit.
I just milled 2mm off of the bumper, but you are right, using a couple of 2mm spacers would work as well.