Note: The image of it on the mattress just shows where it goes. The sheet would go over the mattress and underneath the holder.

I had 8 of these and lost 7. So I designed and printed replacements. They slide over the lip of the mattress from a corner when the sheet is on. It's a very tight fit so you might need go put some effort into it. I printed it at 30% infill for strength.

Creality

Ender 3

No

No

0.2

30