Once we received our mattress in the mail, we opened it up and gave it a full day to expand.

Using a tape measure, a drywall T-square and a sharpie, we laid out the cushion sizes above and cut them to size using an electric carving knife. 

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Note: be sure to make the cushions a good snug fit between the walls. We actually cut our cushion widths at 17-1/2", instead of 17-1/4". While its hardly noticeable, it gives a good compression fit and keeps the cushions from being able to seperate underneath of us while we sleep.