As a side note, there are templates for the Mustangs and Messerschmitts bases listed on their yahoo group.
You can use that, if desired, and produce to any size.
I'd go with a thicker, heavier, probably weighted base, just to make sure the whole thing stays upright, when models are suspended at 6' high.
You can have them custom cut by someone out of wood, or plexiglass. I think I'd go with 1" to 2" thick material.
2" thick plex will be crazy expensive too, but virtually indestructible.
IIRC, Reviresco used to sell the M&M trolleys in two sizes, cast from lead.
There's a little, tabletop model for 1/300th aircraft, and a larger one, both with metal bases. I doubt even the larger one would work with 6' poles – it's probably better for 1/72 scale aircraft on a tabletop.
If someone would make one master, and a mold from it, casting it in lead would be ideal, since it would provide the stability required, but again, it would be crazy expensive to buy, not to mention probably cost prohibitive to ship.
Even the tiny and little bases mentioned above were pretty expensive. The kits slightly less so.