Bent up a piece of 19 ml alloy pipe to hold the mattress in place and stops the Dooner from sliding down, fitted this under the bed base with 4 conduit saddles, problem solved.
Is the sliding down is happening during travel or whilst sleeping? You are not very clear.
If you are losing it over the end whilst sleeping, the best way of slowing that down is to sew some sheeting to the end of the doona caver and tuck that under the mattress.
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PeterD Nissan Navara D23 diesel auto, Spaceland pop-top Retired radio and electronics technician. NSW Central Coast.