The Magic Circle Against Evil spell notes that -
If the circle of powdered silver laid down in the process of spellcasting is broken, the effect immediately ends.
and that -
You can add a special diagram (a two-dimensional bounded figure with no gaps along its circumference, augmented with various magical sigils) to make the magic circle more secure. Drawing the diagram by hand takes 10 minutes and requires a DC 20 Spellcraft check.
Are there any rules for permanent Summoning/Binding circles?
Could a caster carve a "special diagram" out in a stone floor and pour molten silver into it to make a permanent and (fairly) unbreakable circle?
Could a hoola hoop of silver be carried around as a portable ready-made summoning circle (maybe with Shrink Item to make it portable)?