I am an NICU RN and we love to get quilts to cover the isolettes. This may sound strange. Make it black white and red. This is good for visual stimulation. There is a fabric I am using to make visual stimulation blocks to be hung inside the isolettes and cribs.
Here is a link-
http://www.michaelmillerfabrics.com/...=first%20sight

The quilt can have the visual stimulation blocks faced out at first and later as the baby is older can be reversed so they face in and give the baby something to focus on.

That size should also make a good "play quilt" when the baby goes home.

Whatever fabric you use to make the quilt, use a dark color for the backing. The reason we cover the isolette is to protect little eyes which should not yet be exposed to bright lights. Lighter color backings seem to reflect some light into the isolette from the areas not covered.

Hope this helps some :)