I found a core in my junk box that worked for the "Designing an interesting inductor" Here's the physical shape.
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As you can see I dropped it and chipped the most important part off and now it has an air cap. But, importantly, I found with only 60 turns on the dc coil I could reduce the inductance down from 100uh to 4uh with under 500ma. That was using my 100kHz inductance meter, so next I put it on the Q meter and found it runs out of steam at about 300kHz. Q goes to nothing. The AsubL of that core was around 2800. That core is about 2" per side and 11/16" deep, I don't need or want anything that big, but I do need room for enough turns of larger wire to keep the resistance low.
Where can I buy an ferrite E core that has that shape in a material with a 2000 or 3000 AsubL, that will work up to 4Mhz, or even higher?
Mikek
PS. I expect any geometry that necks down to a small cross section will work.