I mainly use the glass from an old B+W television (about 10mm thick!), about 18"x24", but I've also just acquired a thinner and smaller piece of toughened glass that used to be the platform of our bathroom scales (the electronics broke). I haven't used it yet, but it's flat, should be very tough and a handy size for Scary Sharp.
I stick down the wet+dry with Scotch (3M) SprayMount. The TV glass has become quite scratched down the years. I think it's loose chips of carborundum that get trapped when doing things like flattening operations. The scoring is very shallow though, and doesn't seem to affect it in use at all.
Glass is really easy to clean. I use either meths or xylene usually after I've peeled off the wet+dry, and it comes up clean as a whistle. I'd guess you couldn't do this with Corian as one or both liquids would probably melt it. Xylene is nasty stuff so only used once in a while for a 'deep' clean, and out of doors, too. Meths cleans off Spray Mount very well.
You can cut glass easily with a diamond stone, so if there's a window fitter who does shop fronts nearby, they might cut you something the right size from an offcut of plate glass, which you can then make safe round the edges yourself. Shouldn't be expensive, and much cheaper than a granite plate, unless, again, you can find a worktop offcut.