I skipped Masonite, even the 1/8th inch thick version, after trying to bend it.
I wound up going with vinyl flooring panels from Lowes. They come in ugly colors in 4x8 sheets, and Lowes was happy to cut them into 12 and 16 inch wide strips for me.
I notched them around upper deck support struts and arms with a sharp utility knife or my jig saw and was very pleased.
Instead of blending seams with spackle or joint compound, I used a glue for floor boards that holds instantly but allows about 30 minutes working time, and glued curved thin vinyl flashing into the corners to cove each one with a minimum of fuss. MHR had an article about this back in 2010 I believe.