There are a number of commercial moulds, dams and rings to contain pot and screen melts.
You can, of course, make your own. A simple one is to use 10 or 15 mm fibre board to contain your pot or screen melts. Cut the size and shape of hole you want into the board and that will contain the glass.
You can place this directly onto the kiln washed shelf. No fibre paper is absolutely required unless you want to. You can weight the board by placing the supports for the screen or post directly onto the board.
If you want to use the board more than once, you need to harden it with colloidal silica and fire it. Then you always need to put a separator on it at each firing to ensure it does not stick to the glass melt.