You can shim the bottom of the Z axis, here’s a forum where it’s discussed pretty well: Squaring Z-Axis - #4 by RobertA_Rieke
I had to do the same thing with mine, except I had to shim at the top. I just used slices of business card, as shown in this picture.
After you get it square, you will want to use a large bit like a wasteboard cutter (3/4") to help get it perfectly square. Just mill a pocket 6"x6" or so, .025" deep, and if you see a sawtooth pattern then adjust the shims as necessary. Once you get a level cut with a wide bit, you’ll never be able to see any kind of un-square with a small bit.