Stainless steel is sometimes called the almost the perfect mould material. It is lightweight, difficult to deform, and durable for a very many firings. Simple bowl forms are relatively inexpensive to buy — you can even use cheap stainless steel bowls. All you need to do is drill three or four small 1.5mm holes in the bottom for air to escape.
It often is a good idea to fire the mould to working temperature (say 650C) before attempting to kiln wash the form. This burns off the protective oils from the steel. Alternatively, sandblast the mould to clean it and give a small tooth for the kiln wash.
Stainless steel moulds do need to be covered with kiln wash. This is difficult to do when the mould is at room temperature, but it can easily be accomplished by heating the mould to around 150C, then brushing or spraying on the kiln wash while the mould is hot. The water in the wash will evaporate rapidly, leaving the protective elements behind. If you heat the mould too high the water will boil off, leaving gaps.
Also, it’s important to realize that steel contracts more than the glass. This is the opposite of ceramic, which contracts less than the glass. As a result, slumping on the outside of a steep stainless steel form generally works better than slumping on the inside.
Still, you can get away with slumping inside gentle bowl forms; just make certain it’s well covered in kiln wash. A sprinkle of a little kiln wash powder inside can also be considered. Be aware that slumping inside deeper forms may not work.