AFAIK the only brand that routinely uses spherical washers on disc brake calipers are Avid. Think they call it "Tri-align". And that's on Post-Mount, not IS. Or, if you're using an adapter, on the part of the adapter that has the Post-Mount interface.
Never seen an IS mount use spherical washers. Can't see how that'd be possible w/o upsetting the specification way beyond tolerances.
The only disc brake facing tool I've used was for IS mounts. If I had to improvise one of those I'd buy a special machinist drill bit used for drilling countersunk holes for screws with cylindrical heads. They come with a non-cutting center tap to keep them centered. Then I'd mount that in a handheld drill, aim carefully and go at it.
If I was feeling scared or cheap, I'd cut myself some semi-circle shims out of a soda can, insert under the "low" side and experiment with those.