The custom set is the only way you could do the free form Mondrian quilts in EQ. I think you could do better with graph paper, following some of the designs that you will find below.
Some of the Mondrian quilts are actually same size blocks that just look different.
https://www.pinterest.com/sonalees/mondrian-quilt/
There is a lot of variation with this quilt.
https://www.pinterest.com/pin/11329436537694651/
This one shows the different sizes of the blocks.
https://www.pinterest.com/pin/154740937177111867/
Modern City blocks is another example.
http://www.lynbrown.com/modern-city-blocks-quilt/