The older flat-style (3rd, 4th, 5th generation) Airport Extreme and Time Capsule can get pretty warm, especially if they are stacked together. One set of these will allow extra airflow underneath, and more sets will allow additional units to stack on top.
For the bottom unit, put three small rubber feet in the bottom recess, to keep things from sliding around. I use https://www.amazon.com/gp/product/B001JTFPFQ but pretty much anything slightly taller than the 1mm recess will work.
For additional stacking units, leave the recess clear and it'll allow the units to stack.
Inspired by https://www.thingiverse.com/thing:737978 but that didn't do exactly what I wanted, so I redesigned it from some measurements taken from Joe's Thing.
Yellow, Cool Grey, your choice
The support for the recess peeled right off in my M200, YMMV.
I'm including the Sketchup file in case you want to fiddle with it.