Excellent serving suggestion.
However, if you can't get all of the ingredients, you could try a kinesis advantage. It has a habit of leaning back if you use it on your lap, but you could rig a tray and attach it to the base of the keyboards by inserting dowelling into the screw wells (or whatever they are called). There used to be pictures of this on GH, but I doubt they've survived.
Did you mean chair arm mounted?