I need to upgrade my table surface on my Shopbot. Right now it's an MDF vacuum fixture clamped to the main table surface which is 1/4" mild steel plate bolted to 1.5" rails.

The MDF's got to go.

I use a combination of edge clamping and vacuum clamping so am wondering about using either some cast aluminum tooling plate probably drilled and tapped on a grid or T-slot table. Vacuum jigs would get clamped onto the main fixture plate.

Minimum dimensions are 16" x 45" max dimensions are 24" x 48".

I'm mostly routing hard woods, but do a little light alu machining occasionally.

Any thoughts or suggestions?

http://www.lowcostmetals.com/
is a good place to get cast aluminum plate.

I have installed a grid tapped table on my machining centre. Only once in 2 years have I wished for the T-slots.
I really like it.

Where can you find a tapped plate for a reasonable price?

Not usre what you think is a reasonable price, but I recently bought a couple of tapped aluminum plates from The Little Machine Shop.com.

The holes in one are 1/4-20, not sure about the other.