There are mold manufacturers
There are mold manufacturers who make tile molds. Slumpys has some of these:
They are wholesale, I believe, but you can probably also find them at Delphi glass and other glass supply places on-line.
An alternative to this is the make a dam of some sort that is squared up. A dam can be made with “hot damz” which are made for this, or you can cut Kaiser Lee Board into a square, and cut out a square in the middle.
Another option is to square up the piece after it is fired, using a grinder or a flat lap.Flat laps are pricy, but you may already have a small grinder in your shop. Since the tiles will be framed (with wood?) you wouldn’t have to worry about polishing the edges after grinding. If you do need to polish, you can do a fire polish in the kiln, which if done quickly will not allow the edges to flow out of shape.