I just had this dumb idea while reading this. Print a paper boarding pass for the empty seat and just lay it on the empty seat. If anyone comes up to take the seat, hold up the boarding pass and say, "I don't think your boarding pass says 10E, because I have it right here."

Or just put your bags on the seat.
“Protecting” this seat is a mostly artificial problem.

And then have to deal with the FAs telling you to stow it.

No, a people problem.

David

Can you buy XTRASEAT in a domestic F cabin to have an empty seat next to you?

Yes. I have done that before in domestic F. No problems.

Looks like United are making it easier to book extraseat:

I've purchased an EXTRASEAT many times on domestic flights, but I've never tried it on an international flight. How would you check in the EXTRASEAT passenger without a passport?

Using plus points when booking an extra seat....
 

Before covid and now travelling with a +1, I usually would buy my ticket in F so never had to worry. F fares have gotten so expensive for the routes I fly that my fiance and I have been playing PP roulette and losing more often than we win.... We are both GS, by the way and still lose the game :( Today we had exit row with an empty seat which, in all honesty, was not bad at all.

So, my question is, if I want to book an extra seat as a bit of insurance to have extra room in Y, and also still put PP on a reservation for the two of us, how would that work? I assume me, the extra seat and my fiance would all be one PNR.

Book the extra seat on a separate PNR or book it all as one and have them divide it off, then apply the PlusPoints request.