Wow, when I started reading this post I was sure I would be too late to join in. Great stuff, although my head is spinning after reading 17 pages!
Please put me down for two plates in mild steel and three sets of rings in aluminium.
I've never tapped a thread into a hole before - how hard is it to get right?
Also feel free to call me Mr Thickie, but am I right in assuming you mount the router by drilling out the appropriate holes, countersinking them from the top of the plate and then screwing through? Those holes should not be threaded, right?
Finally, being the owner of an old Land Rover, I have to mention galvanic corrosion for those intending to mix mild steel and aluminium as I am. Having the rings sit slightly below the level of the plate will be good as it will make room for a layer of paint on the rings before I shim them up to level.
Thanks
Ian