This is what I use. They stack nicely, and you can even remove the center divider and store the larger format slabs. Each box will hold about 120 slabbed cards or 60 on each side.
(5) Graded Shoe 2-Row Cardboard Storage Boxes - Baseball, Football, Basketball, Hockey, Nascar, Sportscards, Gaming & Trading Cards Collecting Supplies by MAX PRO - GSB
https://a.co/d/1od5emu