Install VirtualBox while keeping Secure Bott

I am trying to install VirtualBox on Ubuntu 16.04 while keeping Secure Boot.
When I installed it though Synaptic, I was asked to remove the SecureBoot, I said No.

I followed these instructions:
Could not load ‘vboxdrv’ after upgrade to Ubuntu 16.04 (and I want to keep secure boot)
and
https://stegard.net/2016/10/virtualbox-secure-boot-ubuntu-fail/
Both are pretty much the same (I left the MOK files in the /root directory like in the second link).
All seems to work fine, I rebooted, put my password again, rebooted. All good

But then when I try to use VirtualBox it still wont work.
If I run it from terminal I get:

WARNING: The character device /dev/vboxdrv does not exist.
     Please install the virtualbox-dkms package and the appropriate
     headers, most likely linux-headers-generic.

     You will not be able to start VMs until this problem is fixed.

But those two packages are already installed and up-to-date.

On of the comments in the top answer of the other post says to reinstall virtualbox-dkms before following those instructions. I tried, and same result.

I tried the answer here:
Problem with the installation of VirtualBox
Which prompts me again to ask if I want to disable the Secure Boot, to which I say No, and back to square one.

If I run modprobe I get:
modprobe: ERROR: could not insert 'vboxdrv': Required key not available

Any idea on how to get VirtualBox to work with the SecureBoot enabled (please refrain from telling me to remove it…)?

thanks