Mortster was kind enough to donate an STI splitter to see if I could cut it about to fit my wagon. Turns out the saloon and wagon bumper really aren't that different. Just needed a bit of filler, bit of strengthening, some colour and new holes drilled et voila:
Granted it's probably not meant to stick out so much at the sides but it I think it follows the lines of the side skirts really well. Very pleased