Sorry, but i have already found and read the links you posted but could not figure it out how download the sysupgrade-file.
when I do “owut download --keep -v” owut finds the right directory but the wrong image-filename:
Image-URL https://downloads.openwrt.org/snapshots/targets/mediatek/filogic
Image-file openwrt-mediatek-filogic-glinet_gl-mt6000-squashfs-sysupgrade.bin
but seems to download a modified version (with other packages than default).