The best replacement for slats is box spring.

The best alternatives to slats are rated as following:
★★★★★  box spring
★★★★★  bunkie board
★★★  ensemble
★★  leading edge flaps
★  box springs
★  slots
★  droops
★  divan
★  slats
  vortex generators
  slabs
  krueger flaps
  no slats

Your alternative for slats is not listed? Suggest!
Speaking about slats, one probably shouldn't forget about metal bed frame.

Other search terms related to slats and their ratings are:
★★★★★ metal bed frame
★★★ box spring
★★ metal frame
★★ memory foam
bed
You have in mind a companion for slats and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of slats in all years. Click on graph for more.
slats
box spring
bunkie board