Josh,

You can still do the corner piece. When you take it out and remove the duct tape from the egg crate and then spray foam the egg crate in place. You can also put a piece of cardboard down the backside to keep any foam for leaking through as it cures (if you think you can easily remove the cardboard once the foam cures. Another option would be to replace the angled eggcrate with a piece of PVC pipe. Fish your lines through it then spray foam it in place

The only issue I foresee with either of these approaches is if you need to remove one of the devices due to a failure you would be hard pressed to feed the power cord back through it.... though as I am typing you could (in the event of a failure) tie a piece of string to the power cord as you pulled it out of the tube and then tie the new devices power cord to the string and pull it back up through the tube