As a guest at this forum I would like to draw your attention to a book I wrote about the making of flat and round sennits:
https://issuu.com/telfort21/docs/handboekIt features over a 100 sennits, of wich more than a few original. All sennits are accompanied by text, a diagram, a table and a photograph. Moreover, the book gives novel insights in the constructions of sennits and describes several new designing techniques.
It is free for everyone to read and copy. A drawback is that it is written in dutch with limits its accessibility to people considerably. Having found new fields of interest, I unfortunately lack the time (equals money) to translate it to english at this moment. Hopefully I will be able to do so in the future.
My intend is to share this information freely with all people interested and your assosiation seems to be the place to find them.