Open seating on an airplane is a blessing and a curse. It's great when you want to save some cash, but the press of humanity struggling to get adjacent seats while avoiding weirdos and the middle seat can be unsettling.
Be one of those weirdos by using the barf bag early in the flight.
Fill the bag up so it looks puffy and full.
Set it on the seat you'd like to keep empty.
Look ill. Point at the bag occasionally.
Watch other passengers sit next to crying babies to avoid you.
This won't work on full flights, but it can be nice when there are 13 open seats and you want to have elbow access to two armrests for the duration of your journey.