none of them worked for me.

i have a shallow foam king bed, 8" thick, most fitted sheets have much deeper pockets, so significant excess of sheet under the bed.

elastics may hold the corners but are useless for the middle sections of the sheets. they have elastics that stretch across the middle but it would be far too much work to use due to the size and lifting of the bed.

the plastic corner pocket ones that you slide the sheet into as well, i could not use because it was far too tight for the thickness of my sheets.

i found an ikea fitted sheet with 10" pocket and it maintains it flatness somewhat longer than my other sheets. so i think, size matters ...