I have had this in my to do list for a long time, and finally got around to buying some polystyrene beads. It didn't take long to sew...took quite a long time fighting with the 'beans' to get them into the bag, they go everywhere...I am sure there is a better way to do it.
I made a funnel from card and used a glass jar pour in. I kept a box by me to place the jar when not in use, keeping the mess to a minimum, I got there in the end, and am pleased with results, you can rest ipad/tablet/ or book in any position, on your lap on the settee arm etc.
Needed to get the vacuum cleaner out, but wasn't too bad!