Saw this Clairefontaine notepad at the LI Pen Show in March and was really taken with the polypro front and back covers in five really bright colors.
At 8.5 by 12, this top-bound double spiral notebook has a very substantial chipboard backing; the ribbing in the covers adds some stability as the front and back interlock and therefore don't allow any sliding.
It's as if you have a double thickness of board to hold the notepad steady, and it works quite well as I wrote most of the below at about an 80 degree angle.
The 80 sheets of bright white paper in Clairefontaine's signature 90 gsm weight is everything you want to write on: smooth, easily readable, fountain pen friendly but not overly so (the ink didn't take forever to dry).
If you have to write using your lap as your backboard there are two notepads to consider: this Clairefontaine Linicolor (which you can find in a medium size as well as large) and the Field Notes Steno Notepad. Both have extra-strong backing for non-desk writing, so the question is whether you want bright white notepaper or steno-style paper.