I got 1/2" GPO-3 fiberglass laminate (industrial grade polyester/glass layup) from these guys for way cheaper than I could have done a layup myself:
http://www.onlinemetals.com/merchant...64&top_cat=181 The only problem is that it is red - messes with my expectations.
You might want to consider a 1/2" Corecell core with solid glass at the hard points. All of that is exactly the same thickness and plays nice together. Another thing to consider if you have small hard points is getting some 1-1/2" solid FG rod, cutting it to the right thickness with a chop saw, and using it as a solid glass plug in 1-1/2" holes from a hole saw; it fits perfectly.