There are various ways of damming oval shapes in kiln forming. Some of these are outlined here.
One set of methods depends on having a soft surface such as ceramic Fibre board or vermiculite.
You can wrap your shape with fibre paper. For this you need to cut a strip or strips 3mm narrower than the height of the piece you are wrapping. You then stick sewing pins down through the fibre paper and into the shelf of fibre board or vermiculite. This will be easiest if you use 1 to 3mm thick fibre paper, as the pins must not contact the glass – the pins will stick to the glass if they do.
You can cut a form out of ceramic fibre board and use that as a dam. You can pin this to the base fibre board or allow it to merely rest on the board. It is possible to cut arcs from fibre board and place them around in sections. In this case they will need to be pinned together so they do not move apart. Staples can form the attachments. You can make your own – larger – ones from copper wire.
You can buy stainless steel banding which needs to be lined with any separator – batt wash or fibre paper.
You also can layer fibre paper up to the height required – remember 3mm less than the thickness of the piece. You then need to fasten the layers together to avoid movement between the layers.
If you are firing on ceramic kiln shelves the same materials can be used but need to be supported a little differently.
If you are wrapping the piece on mullite shelves, use some pieces of kiln furniture to block the strips up against the glass. The thicker the glass, the more weight will be pushing out against the dams and the sturdier the dams will need to be. Make sure the strips contact the shelf evenly- if you have gaps, you'll have leaks.
The disadvantage to this method is that the glass can take up the irregularities of the kiln furniture.
You can use fibre board with a void cut out to the shape required and place it on the shelf.
You can also use layers of fiber paper around the shape and pin the layers to each other. This is the same method as used on ceramic fibre board.
Again stainless steel can be used to form the dam. Remember to line the steel with fibre paper that is 3mm narrower than the height of the piece.
In all these cases of dammed forms, the edges will be of varying degrees of roughness and some cold working will be required.