It doesn't need to be that complicated. I bought 2 - 2"x10"x8' pressure treated boards, then cut them up into pairs of increasing lengths with 45 degree miter cuts on each end. Height adjustment available in 1 1/2" increments. The O.P.'s problem would have been solved with a stack of 3 under one corner and 2 at the other.
For measuring levelness, I use a small torpedo/bullet level, about 6" long, that easily fits on the refrigerator shelves in both directions side-to-side and front-to-back.