|One of the many styles of commercially prepared moulds|
An alternative to buying moulds for slumping bottles is to use a sand bed. You can place the bottle into the sand and roll it a little from side to side to create a depression in the sand that then becomes the mould.
I use a fine sand (not builder's or garden) and coat it with alumina hydrate (slaked alumina). I use about 1 part alumina to 5 sand, but the mix is not critical, just enough alumina to coat the sand particles. It can go directly on your kiln floor if you have an easy way to pick it back up, as it is re-usable. Or you can put it in a stainless steel tray or any open topped box that will withstand the temperature.
Make your depressions and then sprinkle or sift a fine layer of alumina over the area - I use an old sock to hold some and dust it over the sand. Then lay the bottle in the depression.
The amount of sand impression you get is dependent on the temperature you use - the higher, the more sand texture you get.
This way of slumping bottles eliminates the need for a mould and it is variable for different sized bottles.