I've created a patch and want to share it with you.
Because I'm new to docx4j I'm not sure if it is a pratical / neccessary one, but I know you'll tell me.
I created an interface for SdtRun, SdtBlock, CTSdtCell and CTSdtRow, because I've written a utility for my application which handels all of these types. To reduce some lines of code I found it helpfull to create an interface.
All JUnit tests worked for me.
I've read the terms & conditions for providing a patch and accept them.