Mora Companion Firesteel Holder
This is such a good idea I don't know why anyone hasn't done it before. It is made for the most popular Mora knives, the Companion. It will fit all Companions including the Heavy Duties and allows you to combine two very useful tools into one.
They are 3D printed for us in the US out of durable ABS plastic and allow you to carry a range of sizes of firesteel up to 12.5mm (TBS Fatboy). You may need to use a piece of shock cord to secure your firesteel in place (picture 5)
It comes with 2 firesteel rod spacers to fit different sizes and if you find that the size or shape of your firesteel handle interferes with the insertion of the rod, insert the next bigger sized rod spacer to give it more room to articulate. Alternatively you can mount the firesteel in reverse and use shock cord to secure it.
They fit extremely tightly the sheath and do not require any glue. Of course, you could glue it if you so desire to give you an extra level of security.