First if all, good choice. I love the two the two I have. First, there is a bottom 1" layer of foam. Then two pluckable layers, about 4"-6" each. Lastly, a top layer of foam, again about 1" think. Hope this helps!
Since the case is right at hand, I just checked the interior dimensions. I got ...Top: 1.75" deep x 20.25" W x 15.4" H
Bottom: 7" D and same W and H. If you anticipate using the entire interior vol. - note that there are 3 small bump-ins in the inside for the 2 wheels and for the indent on the pull-out/roller handle. Wheels are ~2 x 2" and the handle bump-in (at the other end of the interior, is ~4.5" W x 1" x 1". The first foam layer has cutouts for these. Then, there are 2 separate honeycomb foam inserts at 3" deep that you can customize. The top has its own 2" foam with foam "fingers." I customized the initial foam insert to transport a valuable 400 yr. old marble bust via. air with no problem, and a second set of foam inserts for expensive computer electronics. The case performed flawlessly.