After much discussion on the forum about core size and material, the conclusion I have come to is; as long as it is ferrite and of a similar dimension it will be OK to try out.
Romero stated his cores, for the device shown, are 6mm dia x 15mm long.
As I have some blank AM radio ferrite rods to hand I have decided to go with these as a first-try core. I have 10mm(with small flats either side of 8mm) dia x 15mm long.
If you are cutting or grinding the ferrite make sure to wash them in white spirit after to clean off any dust and grease before glueing to the end-cheeks
