OK we are getting a bit of track here so I just want to rein everyone in for a second.
I am going to do a prototype as soon as I am able (house projects come first!). It will be for the Planar 3, and should work for P3 and other tables. It will use the single hole arm mount in the stock location (222mm from the spindle I believe). The reason is, I plan to use my plinth as the template, so the locations will be as accurate as the original- I will not be measuring and altering the locations at this time.
In the future, assuming things go smoothly, I will entertain 3-point mount arms (newer tables) different spindle to pivot distances and the frames for the framed tables.
And, finally, I'll let the cat out of the bag on my plan:
Plinth 1 will be the old growth knotty white pine I spoke about way back. I have lots of it, so it is 'free' to mess up on, as opposed to other woods I have to pay for. It can also make a nice core for laminates of wood veneer later.
To this, I will apply an upper and lower edge treatment-it will be about 1-1/2 inch thick in the center (where the spindle is) with a 1/8 inch step to a taper that is about 1-1/4 inches wide and then straightening back out. I think this treatment will make the table ~3/4 inch at the outermost edges. The first step will be just outside the dust cover perimeter, so that will all look normal.The back edge will be flat to accept the hinges if you choose. For those who know one, it will be a raised panel shaping- but on both sides rather than the typical single side.
Basically, it will have a sort of frame integrated into the one-piece plinth- arguably the way Rega should have done the high end tables in the first place.
Also (in the future) I can use MDF and veneer that to have a density and sound close to that of the original. Again, I want to stress, these "upgrades" are first and foremost cosmetic ones, and I make no claims to the sonic improvements, if any. With that in mind, as we progress here, I am open to things like the suggested solid walnut above or more economical walnut veneer onto a less expensive substrate.
"Just because I don't know what I'm doing never stopped me before!"