I might suggest a different method. I find eggcrate is expensive and somewhat difficult to cut and fit as well as being a bright white which I find shows more than other materials if/when it gets exposed. I find it a commonly used item but do not use it very often.
I like to make double use of tank space and that often means I want height but also want to use the space below if I have fish who want to hide. That leaves me looking for things to hold the sub up but also let me use the space below. Have you looked at all the other plastic we often throw away or recycle? There are some really nice plastic jugs and cartons which are sturdy to hold the sub and plants, making caves underneath. You may not want/need the caves so that can be used or ignored as fits but the sturdy plastic does seem to work better for me on other points. I use silicone, etc. to glue some of the sub onto the plastic before placing. Doing that makes it less obvious if it gets exposed as the glued portion will blend better than the garish white of eggcrate which shows as totally "odd" if exposed. I find plastic trash as cheap, easy to cut and comes in all kinds of types, sizes, and shapes. That's lets me try dozens of plans which better fits my method of trial and error?