you seem lost & don't know what your talking about, you can create a fud downloader with literally 2 lines of code in c#You'll never find something undetectable bro.
FUD doesn't exist and even if you get a semi-fud payload, if you want to do mass spreading it will prompt runtime in a couple of minutes
To bypass longer than 2 minutes, just make it polymorphic. there is tens if not hundreds of polymorphic samples in the wild