Okay - I'm building everything on top of a base slab of aluminum. It's 12" x 12" and 3/8" thick. I was going originally for 1/4" thick but 3/8" was what was avalable on ebay so I went with that. Here is a pic, it's Mic-6 cast plate:
This will eventually be drilled and tapped so I can attach a vise etc.