I have 2 areas, that I use, to work on mowers. I run a commercial lawncare service, so I know of what I speak. First area that I use is the foundation/slab of an old chicken coop, built into a hill, I can get a range of heights, for different jobs, on different machines, just by where I park the machine. The one that I use most, is a 4' long, by 18" wide, by about 18", skateboard "drop in" tamp, that I found in the park dumpster. I use it to change blades, just drive one of the front wheels up the ramp, set the brake, and chock it, blade change takes about 15 minutes, MAX.
I did notice that the OP, has sense enough to use the cinderblocks the right way, with the holes up, and the board on top, to lessen stress, that might crumble them!