go to your local kitchen supply place, see if they can do you an offcut of granite worktop where they've cut out for the sink. i dont mean a b&Q etc as they don't do it themselves but template it and have it done elsewhere...you need to find somewhere that actually supplies the granite and does the cutting... I know of one locally to me, Planen, at Staples Corner... they have this huge CNC cutting and polishing machine, impressive to watch. I got a 600 x 400 piece of granite 40mm thick. It really needs to be mounted in a frame to stop it twisting for ultimate accuracy, (a real granite surface plate is 70+mm thick) but its good to 0.1mm as it is just sat on the bench...

I've seen the same done with 20mm plate glass but not so large obviously...