I sanded, with 220-C grit sandpaper, the edges of all the sides and cutouts.
On the front face I sanded, with 36 grit sandpaper, the locations where the stiffners will go.
I vacuumed the dust and cleaned up the shop, then began cutting the foam for some of the stiffners. The stiffners are made from 1" wide by .2" thick foam that is positioned on the front face of the instrument panel and once glassed will provide additional strength and act as a conduit run for wiring behind the instrument panel.