The supports for aperture drops need to be rigid at tack fusing temperatures. A number of materials are rigid enough to maintain their form. Those such as ceramic, or fibre board are commonly available. The ceramic forms can be purchased from various suppliers. Fibre board can be carved in a number of shapes and so are more versatile. They are more flexible than ceramic so need careful support.
The supports also need to be of such a material that will not trap the glass when cooling. This makes metals unsuitable for use as drop supports. The metal contracts more on cooling than the glass does, and so traps or crushes the dropped part of the glass.
Note that the supporting structure does not have to be flat. It could slope toward the centre, or could be curved down on the outside. The permutations are up to your imagination.
The other element of support is the material to hold the support surface above the kiln floor. These supports need to be stable so should have a relatively broad base in relation to the height of the support. Two good kinds of supports are kiln posts and fire brick sawn to the appropriate height. There other possibilities to create home made kiln furniture. [qv]
Note that it is important to kiln wash all the supporting materials to avoid any glass getting stuck to them.