What is a "COTS" Core ?
What is your motivation to DIY a new core shape?
We found the existing core shapes are fine, and bobbins TH, SMD are generally available
If you DIY a custom core, what about the bobbins?
Have used RM, EER, ER, PQ, UI, UU, in designs SMPS, EBU, HV, pulse.
What's the intended market or,application ?
Power thru put ? frequency min/max ?
Jon