[ Ref. John A. Tuttle's article in 070214 MMDigest ]
You say that the microwave method worked on the smaller pieces. You
also say you are using freezing on the larger pieces, I assume because
they won't fit in the microwave. If freezing does not work, here is
Hook up the output of a steam kettle (or large metal gas can with water
heated to boiling) with a tube to an enclosed chamber to heat up the
wood. For the enclosed chamber, I would suggest a large diameter piece
of PVC pipe. I saw this technique demonstrated to heat wood for bending.
I don't know if the high humidity will be bad, but it will give you the
gentle heat you are looking for on the larger pieces.