I like the idea of portable protection with firmly fastened edges.
I use chicken wire weighted down with bricks. However, if the squirrels were hungrier, they could easily sneak or burrow under the edges.
I think that I'm deterring more cats than squirrels. Too bad the cats don't chase the squirrels away!
This one is only sufficient to dissuade cats:
This was intended to keep cats away, but was totally ineffective. Maybe if they had been blackberry branches with big thorns, the idea would have worked:
This worked. I just had to remove it or arch it up before the seedlings grew through it too much. I named it Stalag RB 1.