For future travel until you recover, I would say go straight ahead to book the extra seat in advance under the Customer of Size and Extra Seat policy. Customer of Size Information | Southwest Airlines
Southwest's policy: "Customers who encroach upon any part of the neighboring seat(s) may proactively purchase the needed number of seats prior to travel to ensure the additional seat(s) is available." and "If you .... encroach upon any portion of a seat next to you, you need a second seat. ".

Book the extra seat in advance and don't wait until the gate. That way you wouldn't encounter push back from gate agents on if you qualify as a Customer of Size (and avoids inconveniencing another passenger needing to be rebooked if it is a full flight).