I run Odoo12 POS secure server. The POSBOX v (20.02) is also secured via NGINX and a self signed certificate. 

Problem is; Because the self signed certificate, Odoo POS still will show a mixed content error. This is unless one enters the POSBOS via its IP (e.g. https://123.123.123.1) first and accept the certificate.

Only thereafter you can start POS without error message and a running POSBOX.

My solution would be to install letsencrypt (or similar) on POSbox and install a certificate onto the POSbox. But as POSbox image runs as read-only there is no (easy) way to install additional software on the POSbox.

Has anyone solved this issue yet? The webkul solution  (https://webkul.com/blog/serve-odoo-posbox-over-https/) is only a partial solution because if you forget to accept the certificate first it even might result in crashing odoo server (needs reboot).