I put 6 cinder block bricks under the box spring. 3 on each side and 3 along
the middle.

So the box spring is firmly supported and the head/foot boards are just
sitting around this - not supporting anything.

The cinder blocks are spaced out and this leaves little "compartments"
between the bricks under the bed. My dog loves this to go hide under there
and peek her nose out an opening (to keep an eye on me to see if I am going
toward the refrigerator or any source of food...)