insulation material for a diy power transformer?

[Marcos - Munky]

Just got a roll of kapton tape. Now I have to disassemble the transformer to measure the bobbin again and make a new one. I have a few things on my desk I'm planning to test this weekend so I can clear the desk, then I can work on the transformer. Until then, I'll think on how to do a split bobbin and how to improve my very crappy winding machine.

[Rob Strand]

Until then, I'll think on how to do a split bobbin

If you do you own,  you should seal where the added "partition" touches the bobbin.
That makes the creepage distance over the outer edges of the partition, not the short distance along the bobbin through any cracks.
It's going to be tricky to get right so if you aren't using commercial bobbin it might end-up being better to use an overwound design with good insulation between.   Even here you need to consider creepage around the ends of the insulation, perhaps need to use margin tape.

There's some tips on this page,
https://ludens.cl/Electron/trafos/trafos.html