Comment by londons_explore

35mm of silica glass would do the trick.

Since the ferrite core isn't a good insulator, the glass would need to fully encase either the primary or secondary winding.

At the sort of scales this transformer would likely be built, an extra 35mm would make the whole thing a little bigger and more expensive, but not massively so.

The glass tank could also double up as an oil bath for cooling the coil - the first 500 millimeters or so of the piping needs to be glass, but after that you can use a typical cooling radiator with no extra concerns.