C CRYSTAL LEMON 10-inch Bed Bridge: Transform Twin XL to King with Our Deluxe Twin Bed Connector with Comfort, Anti-Slip Design

I am trying to use to keep a daybed with a pop up trundle open as a king bed. I was hoping this would fill the gap. It falls into the crack easily. I tried putting a fitted sheet down 1st then the gap filler, to see if that would keep it from falling down in. That didn't work. It may be ok if the mattresses were side by side, bit doesn't work for what I needed.

The strap does not hold the two mattresses together, and slips incredibly easy