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The best published information on sonotrode design I know of is a 20+ year old handbook produced by ZVEI (the German Electrical Manufacturers Association) "Ultrasonic assembly of thermoplastic mouldings and semi-finished products - Recommendations on methods, construction and applications", available direct from them.
Please don't ask me to supply you this handbook - it belongs to ZVEI. If you'd like a copy, please go to the ZVEI web site and request it.
In this section, by kind permission of ZVEI, I've reproduced pages 33 to 38 (Sonotrode manufacture) - to view, click on the page images or links below.