I have a customizer part on thingiverse to make a block any size you need.
I feel I should put it out there, you should only be shaving off the surface of you cutting area if it is raised above the rest of your machine. When you build it it should be pretty flat already, if you were going to make a small part that needed that kind of accuracy you could do just the are of the part, hope that makes sense.
Now imagine if you had to shave it down (exageration) 5mm. You would be left with a 5mm hole in your cutting area. If you were cutting a larger piece than you cutting area it would need to hang out the side, when you clamp it down it would be worse because of the shaved out area. Dang, Not a good description but I hope it makes sense. All your material will not fit in your level area is the gist.