compound dataset with extra dimension on one column

Dear all,

Not sure I have sent the email to the correct list. Apologize if you have already received it.
I am trying to build a large 2D compound dataset that contains only one column that has an extra dimension (2D). This is the column 'iso_massf' in the snapshot below (using HDFView):

The number of elements in "iso_massf" is dynamic and is determined at runtime.
I have been able to construct the whole dataset without the 'iso_massf' column, but I am struggling when trying to include it. I have seen some examples about variable-length datatypes on the API (\) but I need to have a compound datatype, not a variable-length one. Is there some example showing how to do that?

Any help would be much appreciated.
Thanks a lot,