Thanks for the advice Jonas, I think that might be exactly what is wrong. I took the plunge with the first of five pieces, and the results were a little better than the scrap oak I experimented with, but still not great. The wood did not break, which was a relief, but there is a fair amount of set it still needs to take. After taking it out of the box (pipe!) I used the kettle with the smaller pipe still in it as a steam gun to keep the thin wood from cooling and drying off too quickly. That helped too, but still not quite enough. I’ll grab some insulation and give it another try!
Thanks again!