I have an A5 to A4 kneeboard and I've never once opened it in flight. It won't actually stay open anyway so its not really ideal.
Also, it was intended to be worn on the right leg and written by a right handed person, as it opens right-to-left (as a book does)
I'm left handed so I wear it on my left leg, which means I wouldn't be able to open it, so now I wear it upside down on my left leg which is actually better, as the clip is now at the bottom. I can clip my flip-over check list in it now and still flip it over without dropping it.
But its going to have to go soon, my wrist sticks to it as the stick is quite short. I've just bought an A5 hardboard clipboard from an ofiice supplier and I think that can be modified to perfection.