Find a script that does the job here. It will download the source packages if they do not already exist and then unpack them into a subfolder of the current directory named tmux. Then it will one by one compile libc-musl, ncurses, libevent and tmux. Adjust the URLs at will once updates arrive.
One caveat exists, though: the ncurses library expects a terminfo database. This can be a problem for portability of the otherwise fully static binary.
// Oliver
You can also get a tmux static version with static-get
sh <(wget -qO- s.minos.io/s) -x tmux[0] https://github.com/minos-org/minos-static