I also use something to mark the inner side of the circle, but I don't like poles for it. I ruptured a tendon myself by stepping sideways on something like a pole and if the horses are not fully balanced yet they are bound to fall into the circle and step on the barrier.
Here in Austria and Germany there's something called "Dualgassen" which became rather popular a while ago. It's basically foam strips with a tarp cover around it.Looks like that.
It's soft but still robust and very light to carry around
. I can really recommend something like it. You can just cut it from foam yourself I guess...
Dualgassen sounds like a good idea but my guys would tear it to pieces
cones they pick up and throw about, tape they run through, I guess they are hooligans
Seriously though if you start your horses in walk and click for stretching forward and balance before moving into trot then ground poles pose no threat . It is easy enough to shape them to work one or two feet away from the poles mine work quite wide of the square. I also make my square slightly smaller than picadero size as I prefer to work a square with rounded corners rather than a circle.