Please read this: https://github.com/lgblgblgb/mega65-utils/blob/master/eth-tool/README.md
And the source: https://github.com/lgblgblgb/mega65-utils/blob/master/eth-tool
You can find only (maybe not the latest - you SHOULD compile the source version) binary version here ...
tool.d81.gz is the gzip'ed D81 disk image
tool.prg.gz is the gzip'ed PRG file
Program should be loaded in C64 mode (thus, GO64 first)