Got my prototype iPad Fatsac in the mail. Came with a load of wadding and memory foam - the latter definitely works better. Got a few areas that can be improved upon and it will take a while to work out the right amount of memory foam chunks to use inside to balance malleabliltiy and support but it's working out pretty well as is so shouldn't need too many revisions.
If anyone has any interest in a simple one-piece, parts free means of comfortably using the iPad with both hands in bed, on the sofa, in your lap and so on, that you can step on/throw at people's heads and avoids 'iPad wrist' and awkward typing angles, watch this space...