Thanks for all the good ideas.
This was my methodolgy (for good or bad): 1. used 1/4 " upcut spiral bit to define the edges of the square in two passes of 1/8" each. That worked great. Used a straight edge as a guide. 2. Switched to a 1/2" straight bit to remove the remaining center in 1/8" increments (two passes).
Comments or questions:
bmb: Wouldn't I quickly end up with only 1/2 the base supporting the router as I moved out from the center? A tough balancing job with my Bosch plunge router.
tom: I'm with you on the long rods through the edge guide holes (clever) but you lost me when you talk about two other supports. Please explain.
reible: Thanks for the info on acrylic. Also read your referenced post. Got to hunt around for one of those base making kits. I did think about using 1/2 ply for a base but I would have to make a lot of holes in it to see and I was afraid the rigidity would suffer.
I thought it wise to only take 1/8" passes but maybe I made my problem much worse having 3 depths (surface, 1/8", and 1/4") to contend with. Should I have done 1/4" in a single pass?
Thanks again, Lee