We decided on the layout of our cushions within the "Benches & Flooring" section. See that HERE.
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.
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.
For the fabric cushion covers, we decided on "AquaDuck" outdoor furniture fabric in Charcoal.
The fabric weight is 8oz.
AquaDuck is a product we stumbled upon that is exclusively sold by BigDuckCanvas.com. It is is water repellent, fade resistant, mold and mildew resistant, and is easy to clean. Although it is specified to be "outdoor furniture fabric", we ordered a sample and were surprised by how soft and comfortable this material is.
We sent our cushions and fabric to a seamstress friend to have zip covers made.