The secret with TTS decoders is to NOT use heat shrink, not even loose fitting un-shrunk heat shrink as they do get hot and do need cooling air and get hot enough to shrink un-shrunk heat shrink.
Hornby used to just recommend a strap of thin tape over the decoder to hold it in place and some thin tape under the decoder for insulation, I use Kapon tape and have never had an issue, actually the only time I had an issue was with the first one I tried were I used un-shrunk heat shrink, which shrunk with the heat and resulted in a blown decoder due to no ventilation.
Obviously others will do different but that's what Hornby used to recommend and may still do