Mattresses Foundations Semi-Flex Steel Foundation Solstice Sleep Heavy Duty Steel construction for heavier mattresses also for longevity for those with a more active lifestyle. AKA box spring Profile Height 9 Foundation Twin SFD-1010 TWIN $116.00 Foundation TwinXL SFD-1020 TWIN XL SINGLE $124.00 Foundation Full SFD-1030 FULL $144.00 Foundation Queen SFD-1050 QUEEN $155.00 Foundation King SFD-1020X2 KING TWIN XL PAIR $248.00