Acrylic risres. I get mine here:
https://www.achievedisplay.com/acryl...lay-risers.asp
The 2x2's can be used in multiples for lower tiers, and with items that have larger bases. The 3x3's 4x4's, and 5x5's are perfect to be used to gain height in the back rows. (6x6's are large, but can occasionally work for shorter poses and figures with ridiculously huge bases.)
I have started using them upside down, or even on their side to fit best with each Bishoujo's base as well. You can get a good balance on the cut sides for some of them you can't get on the top/rounded sides that most risers have.
Now, my current display is super packed (with one slot open for Supergirl 2.0). I just don't have room in this house to add the 4+ more cabinets I need. Even my overflow cabinets are at capacity. But you can get really creative and stuff them in there with the risers, they extend your display options immensely and help utilize the height of the detolf's shelves.