This was adapted from the version of widevine-flash_armhf.sh that can be obtained from here.
This chromium-v84-widevine does not download a 2GB ChromeOS image file, but simply downloads the two files you need: /opt/WidevineCdm/_platform_specific/linux_arm/libwidevinecdm.so and /opt/WidevineCdm/manifest.json.
git clone https://github.com/Botspot/chromium-v84-widevine
sudo cp -a ~/chromium-v84-widevine/WidevineCdm /opt
sudo rm -r /opt/WidevineCdm