Interesting, self-signed certificates (those not validated by an authority) would still pose prompts on-screen when accessing these self-signed ip/url... unless like you say... you store it in your browsers safe/allowed database. Tends to be a headache.
You can place the self-signed cert wherever you like, just as long as the key/cert/pem etc. etc. have the proper locations when included in your apache default-ssl.conf file.
You may additionally request via PM (to yours truly) that you have a unique subdomain on the QuickBox DNS (eg; YOURSUB.quickbox.io). This will provide you an A record to point your IP towards and then you can make use of a valid Let's Encrypt certificate.