Entire squashfs-tools in Windows, so you can both "unzip" (unsquashfs.exe) and "zip" (mksquashfs.exe).
Get Cygwin if you don't have it.
Install the following Cygwin packages:
If you're running 32-bit:
gcc-core
cygwin64-gcc-core
If you're running 64-bit:
gcc-core
cygwin32-gcc-core
xz
liblzma5 (not sure this is needed)
liblzma-devel
Get the latest squashfs source.
Uncomment the following lines in squashfs-tools/Makefile:
XZ_SUPPORT = 1
LZMA_XZ_SUPPORT = 1
In a Cygwin console, go to the squashfs-tools directory and compile with:
make EXTRA_CFLAGS="-Dlinux -DFNM_EXTMATCH='(1<<5)' -D'sigtimedwait(a,b,c)=sigwaitinfo(a,b)'"Install with good old make install
Now you have the squashfs tools in a Cygwin command line just like if it was Linux. As stated above, the tools are unsquashfs.exe and mksquashfs.exe.