With this update of Reality Mixer you can have disc shaped portals.
Discs work the same way as boxes. Click the button 'Disc' on Reality Mixer's page on the SteamVR dashboard to create a disc that shows the camera passthrough.
This version also introduces the 'Look-at-me' feature. You can turn this on per box (and/or disc) on the Box Edit Panel when editing the boxes/discs. When enabled, this feature keeps the front side of a box or disc pointed towards you.