Rebonjour ricko_uk
The design of an inductor or transformer includes the power level, frequency ranges, voltages currents etc.
After these requirements are determined by the particular application, the choice of core shape, wire, bobbin (if any) winding and wore layout can begin. Thus is part of magnetics design.
Thus your note ... in order of preference: powdered iron, ferrite, transformer steel.... reveals that more work is needed to specify exactly what you seek to accomplish.
For shapes, instead of toroids, ferrite UU, EI and EE cores are all available with round center legs.
Toroids all have chamfered rect or square CX sec for ease of molding and winding. The Jantzen part may have such a core CX but it is covered in tape, shield, etc.
Core vendors
Ferrite: TDK, EPCOS, Tayen Yudo, Nippon Ceramic, Fair Rite
Powdered iron: Micrometals
Transformer steel: Arnold Magnetics, Magnetics inc,
I suggest you research more deeply you specifications, the field of magnetcs design and the vendor catalogs, then proceed.
You will learn a lot.
Bon Chance,
Jon