Audiobook multi .mp3 files to one .m4b file

[jurak]

Does any one have an audiobook postprocessing script to convert multiple .mp3 files into one .m4b file?
I use readarr to find and send the audiobooks to sabnzbd. But would like to have sabnzbd run a script to convert the many mp3's to one m4b before telling readarr that its done downloading.
I have found many dockers that will do this but nothing that will automate this between sabnzbd and readarr.

[iUseNetter]

How do you actually do this conversion?
What kind of software do you use to convert multiple .mp3 to one .m4b?

Maybe this software has some kind of command-line function that you could use?