Update: see experience report.
Since I am considering using an iMac 5K for my Mercedes Sprinter photography adventure van project, protecting the iMac 5K becomes important, particularly on rough dirt roads; it cannot sit on a desk while driving or it will end up smashed on the floor.
I’m still investigating a VESA mount iMac 5K including products like the Tether Tools Rock Solid VESA iMac Stand Adapter so as to not stow/unstow, but those solutions still will subject it to harsh shock and vibration and so I think they might not be viable.
So I looked around and the best case that suits my needs is the handsomely made about $199 Gator Cases Creative Pro 27" iMac Carry Tote. A big plus is that it is possible to use the iMac without actually taking it tout of the bag—just unzip the front; it has holes for cables to go through as well. I have one to evaluate and will try it out on my next mountain outing.