How to run Windows 10 from USB on my Mac?
I don't think this is an ideal situation to answer your question, but we run Windows 10 in VMware fusion and you can store the files on external USB. It's easy to set up, clear support path, but you have to install software and have a license for it so it's not something you can show up to the Mac and boot - leave no trace behind.
Check out the personal use license for VMware Fusion Player to get started while you wait for someone smarter than me to weigh in with how to do precisely what you ask or explain why it won't work.

https://my.vmware.com/web/vmware/evalcenter?p=fusion-player-personal

Maybe a solution like this would help?