Learning new things all the time.

So I am finally getting around to working on Disk Brakes
and am working with a TRP Spyke, which apparently being a high end brake, doesn't use spherical washers in the mounting.
Hence I have now learned the importance of having your IS mounts be square and faced, so that the caliper can be nice and parallel to the disk.

I'm a growing bike mechanic, but the place I work does cheap hybrids mainly and we don't have disk mount facing tools.
Any advice on how to handle the facing?
Anyone have some DIY tricks on how to do it, or am I going to have to seek outside professional help (ie, a higher ends LBS than my own)?

noted: personal bike, not a customer's. we inform customers who ask, that we aren't equipped to work on disk brakes.