Secure boot cannot be enabled (as far as I know).
There are other steps you can take to increase the security:
- lock the boot loader
- sign builds with your own keys (instead of the google test keys)
- enable verity mode (disabled in LOS)
- create a ‘user’ build (instead of ‘userdebug’)
But keep in mind that security will always be limited by the disabled secure boot and the fact that EDL mode is easily accessible.