The Pressure Relief Overlay Mattress on top of a standard mattress, under the bed sheets.
It is good for helping to prevent decubitus ulcers, but is also used by many able bodied people with bad backs, aches and pains, etc.
Place the overlay mattress on top of the existing mattress with the convolutions facing upwards. This is the part the patient will lie on.
If necessary, use a waterproof sheet on top and then a normal sheet over it.