Base Foundations for Hybrid Waterbeds offer extra support for the top of the bed.  Available in single piece Single, Double, Queen or split (2pc) Queen and King. Looks like a regular box spring mattress but properly re-inforced.

Single/Twin 39×75" $189.00

Full/Double 54×75" $219.00

Queen 60×80"  $249.00

Queen (2pc) 60×80"  $299.00

King (2pc) 78×80"  $319.00