I came across a ready-made base that had most of the features I had been thinking of. Looks like a slab of plywood but actually contains hundreds of small compartments filled with tiny ball bearing sitting on a sand base. Even better, in the centre is a solid area about 100x50mm that allows mounting of the bearing and sensor (otherwise, if you drilled it, all that stuff would flow out!).
So what would have been a major task turned out really simple. I've embedded a thread in the middle, so the bearing is screwed in with the nut acting as a locknut, and cast a cylindrical arm-board to match the platter. This platter/arm assembly is spiked to an MDF/lead/MDF sandwich which carries the motor pod and will sit on a sandbox.
First impressions are that it is very good indeed - and right now there must still be big improvements to be made. What strikes me most is that it's impossible to describe the "sound" - it just doesn't have one. The character changes totally depending on the record I'm playng, the deck itself is totally neutral, even the character of the blank bits between tracks changes from disc to disc.
I'm using an OL RB250 with Len Gregory 'Music Maker" cartridge and an EAR
phono stage, and first impressions are that it's an excellent synergy.
-- Jeff