Simply place your NASlite box in a DMZ on your firewall and use IP based rules.
If you need username and password authentication you will need some sort of proxy as well however i wouldnt recommend allowing access via HTTP to the web.
NASlite has a design principle of being open. This means that essentially there was no reason during the devlopemnt to look at securing anything since it was always meant ot be open. Naslite is also not designed to be public internet facing.
I humby suggest that you dont do this.
However if you MUST do this and see the risk as acceptable to you then i would suggest the following.
Place NAS on a DMZ. On this DMZ place another box. INstall Apache on this box. Remote mount the NASLite drives on the apache box and then present them via apche and .htaccess control. Consider always having secure firewall rules as well.
I do predict though that you will live to regreat this
