We've generated boot.img files in
this thread
I'm on vacation otherwise, I've the android 9 3.0 and 3.1 img-files.
However, without adb how are you going to flash them?
First, I would redownload the upgrade file. If upgrading works, like you wrote, than the culprit could be upgrade file: either the wrong version or a damaged download or a error during unpacking. I didn't find checksums to check, so I had to rely on the archive checksums used during unpacking. (An older WinRAR will generate errors with newer checksums so either use a current 7z or pay/hack a new WinRar.)
Success and keep us updated. I think you'll manage without sending it off.