You want electrical isolation and good thermal resistance, My favorite is the Sil-Pad because it does not require heat sink grease. My second favorite is kapton. I'm not crazy about Mica. You want ow thermal resistance on the back of the TO-220 transistor.
Kapton tape can surely help in the construction. The Silicone adhesive is good to about 200 C. It's also available with an acrylic adhesive. I have both tapes at home. I used the Silicone adhesive regularly at work (semiconductor research lab).
You could probably get away with a larger piece of Kapton tape with silicone adhesive that essentially fills the C-channel and some thermal grease on the other side.
If you read into the details, I would get a TO-220 mounting kit instead of a nut and lockwasher. The bellveille (dome shaped) washer used is superior to lockwasher. see: https://en.wikipedia.org/wiki/Belleville_washer
Kapton tape can surely help in the construction. The Silicone adhesive is good to about 200 C. It's also available with an acrylic adhesive. I have both tapes at home. I used the Silicone adhesive regularly at work (semiconductor research lab).
You could probably get away with a larger piece of Kapton tape with silicone adhesive that essentially fills the C-channel and some thermal grease on the other side.
If you read into the details, I would get a TO-220 mounting kit instead of a nut and lockwasher. The bellveille (dome shaped) washer used is superior to lockwasher. see: https://en.wikipedia.org/wiki/Belleville_washer