However your question was how to make it secure. If you just want to make use of the drive LOCALLY, disable ftp etc. Also if I recall correctly you can set permissions level per user on the same page. Read only, read write, and maybe even block certain users (don't recall).
Yeah, my bad with my post above. Too excited on my end and got ahead of myself
The reason I stated that I am using AD was for the security side of things. A little overkill, I admit, but some of the extended family that use my NAS for backing up data wanted the ability to change the NAS password at will. At least with the AD setup, with Win10 Home or Pro, they can change the NAS password from the Alt-Ctrl_Del -> Change Password screen. Setting permissions also becomes easier with the AD setup as all can be done through ACL from windows.
It was also a fun project to learn. Please let us know what way you went.