Free fret leveling beam.
 

I didn't have the 75 bucks for a fret leveling beam. Ron Kirn, posting on tdpri.com, suggests a piece of scrap counter top. They are amazingly flat. I got a 3.25" x 29.9" chunk of quartz polymer free at a local counter place. Corian and granite also work, but Corian seems a bit light, and natural stones are fragile. I found affordable rolls of self-adhesive 220 and 320 grit sandpaper on eBay.

I wouldnt much trust scrap peices from countertops. Get the correct tool if you want to do a correct job.

If it's hard, flat and can't bend, why isn't it the correct tool?

I agree that it is milled so flat, that it may even be flatter that EXTRUDED aluminum bar. Anyway, the surface of it will be so durn flat it will work very well.

I once skated a 2nd cut file all over the frets after spraying them with Dykem blue to make sure I wasn't taking too much off. Finished 'em up with a Stew-Mac fret file and the frets played great. Files are notorious for warpage, but I didn't see any daylight between the frets and the file before starting (that's a more precise test than might be thought) and I think if you play an average action like I do, there's a decent margin for error available. The whole reason for leveling the frets is to prevent buzz, and with a little relief and average bridge height, we can easily eliminate buzz. Yah, maybe in theory, some strings get CLOSER to buzzing on some frets than others, but a miss is as good as a mile. Anyway I had no probs. I only got the quartz polymer beam instead of using the file again because I want to think I'm going to do a better job this time, though I was totally satisfied with the last one I did.