Agreed Peter,
The edge of the cores can be sharp enough to breach enamel wire. Teflon tape is recommended to reduce this problem. Unfortunately any old tape, some good, some bad, gets used. Teflon has low RF losses. The Teflon tape is very thin and makes no measurable effect on the flux enclosed. Available from plumbing stores.
Flattening the windings against the four sides of the core would have more effect than the thickness of the tape, although I’ve not seen this mentioned.
Insulated wires from CAT5 or CAT6 cable have low losses to 100 MHz. I use these for the primary winding and enamelled wire for the secondary. The extra intrawinding capacitance is less significant on the primary.
I think a compromise between spacing of turns to reduce shunt capacitance and keeping the winding short to minimise flux leakage may be necessary. I generally don’t have much spare winding space.
73
Ron
VK3AFW