The assumption is that these pieces will be open-face thick fusings/castings rather than enclosed castings.
There are two basic types of dams: a shape cut from a single surrounding piece, or multiple pieces held in place.
Single piece dams
A large, thick fibre board with shape cut out will confine the glass. If very thick, you may need to weight the fibre board, as it is lighter than the glass.
Another variation is to use thick fibre paper cut out to shape and layered up to the desired height with stainless steel pins to hold the whole in place. This also may need to be weighted down. A variation on this is to place the whole on a fibre board and pin the layers of fibre paper into the fibre board to maintain the position of the fibre dams. This will not normally need weighting.
Multiple piece dams
If the shapes are not extreme, you can use pieces of fibre board or fibre paper backed up with kiln furniture, bits of broken kiln shelf or any other heavy material that will withstand the heat of fusing.
You can use thick fibre paper held in place with kiln furniture, if the piece is not thick. You do have to be careful that the glass does not float the fibre paper and run underneath, so about 10 mm is the maximum for this kind of damming. It also helps if this kind of dam is made larger than the glass – or alternatively the glass smaller than the dam. This allows the glass to flow out toward the dam, giving nice curved edges.
Moulds, stainless steel and other refractory materials can be specially made for shapes that will be repeated.
Note that all these variations will benefit from being lined with Thinfire backed up with fibre paper. This gives a smoother edge and also gives some cusioning between the dam and the glass.