Glass should work. I used a piece of smooth marble I got at the store for like $5 and put some non-skid carpet stuff under to keep it from sliding on my workbench (ok, it was my coffee table while watching some tv). I also taped the sandpaper to the marble with scotch tape but packing tape is supposed to hold better. I only pulled it out of the tape once after several hours (my 920 was SUPER convex). I started with 220 grit (ended up using 4 sheets cut in half with 400 - 800 passes per half), then 320, then 400, then 600. Beyond 600, I have not found anywhere that shows proof of any thermal benefit, I have found several places that seem to mention making a mirror polish and the thermal paste acting as more of a barrier than a help in those instances. As a result, I only used up to 600 grit. If I could find evidence of any benefit with a higher grit, I'd do it. The key thing you are looking for is flat, not specifically smooth. Also, keeping the surface wet helps the item being lapped glide across the surface. If it skips on a dry surface, you will end up with a slightly beveled edge, which I have on my 920 from having to make so many passes but it is less than a mm so I'm confident the thermal performance will not be measurably affected. Keep in mind, most CPUs seem to be concave and you are attempting to remove a convex surface from the cooler. There is a slight chance your cooling could be worse after lapping, but that doesn't seem to be the case with any level of modern hardware, especially if using a decent TIM. Do all the rough shaping at your roughest grit, and once the whole surface is flat, smooth it out with your finer stuff. Remember to rotate every X number of strokes (more frequent when doing the finer grits) and you should be good.