If it were me, to go that route, I'd buy (or borrow) the Mark Cotta Vaz/Shinji Hata book 'From Star Wars to Indiana Jones - The Best of the Lucasfilm Archives'.
There's a good photo of the original Grail Tablet prop. I'd photocopy it, enlarge it, and start with that as my pattern...
I remember a while ago there was a site that had the full grail tablet that could be printed out on large paper. since i dont have a large printer im stuck
:confused
If you download the PDF above (not sure why others have a problem, works for me every time) it's made to be printed on 8 1/2 x 11. Surely you have that?
I'm trying to think how I could go about making this. I'm thinking many chunks of that green flower foam carved out then painted. Or maybe just a top layer of that about 3/4 of an inch thick over MDF to give it weight, maybe glue and samwhich some weights into the MDF to make it heavy. I dont think it would be that hard. The coating, painting and weathering might be hard though.
Its been a while since I've posted here but I had to chime in with a Grail tablet I made a few years ago using polyurethane foam. Polyurethane foam is more ridgid than the flower foam or the pink insulation Home Depot foam so it will not dent when you are working with it. Here's a few pics of mine:
The flash has washed out some of the details but it should be good enough to make out. Its not screen accurate but it does look pretty good on my living room wall and I'm pleased with how it turned out.