One of the things I made for my mom for her Filofax birthday gift was a board for the back of the planner so there was a more even writing surface. I cut apart a cardboard box and covered it with patterned paper on both sides so it was reversible.
Did I mention when I purchased my mom a pocket planner that I also ordered myself an aqua patent leather one? I couldn't resist.
I actually made one for my full size planner too, since the board creates a much better writing surface.