Why would you pass out? Surely the best remedy is based upon why you may pass out? Is it sitting for a prolonged period or lack of food or the anticipation of pain? Admittedly the longest I have sat is three hours and I just read a book! How are you sitting? Upright or laying down?
Personally if you think you will need it flexi syn the day, work out the best options based upon what you will have time for. They will have to have a break as they will need one as well as you. Take fruit and maybe dried fruit as your hex b and some lower syn snacks like a curly wurly for a sugar hit. If also take a caffeinated drink of some kind, maybe some diet coke or similar. Possibly a light lucozade (not sure of syns but the glucose would help) All of that's portable and can be eaten in the chair. In terms of the fruit I'd opt for bananas for their fillyouup ability.