Fluxbox is a windowmanager for X. Its sourcecode is distributed under an MIT license. Fluxbox was derived from the Blackbox v0.61.1 code.
debian 9 (aka "stretch") repository will contain fluxbox v1.35 (FROM BACK IN 2015!).
Although this custom build, with default menu, keybinds, etc. tailored to antiX, is based on:
http://git.fluxbox.org/fluxbox.git/commit/?h=prep/release-1.4.0
I'm calling it "v1.3.8" so that if/when the official v1.4.0 lands in debian repos, antiX users can just upgrade to the "debian official" packages.
To build an installable debian package, after downloading this source code, refer to the instruction within the INSTALL file