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TIP: The reflex mirror on my Nikon D3400 is spring loaded so that it will come back down under spring power and not under lever power. That means if the mirror is blocked when it goes up, the lever power can break something. However, when the lever comes back down, it is the spring on the mirror that brings the mirror down, not the lever. Thus, they mirror can remain in the up position while taking a photo. Meaning, you can DIY gear that can fully enter the space the mirror normally takes up, keeping the mirror up and safe while using the invasive lens. You will need to use the LiveView LCD screen to compose, as though you are using a mirrorless camera.
I tested this as a theory by using a flimsy piece of folded paper to push the mirror up and take a photo. Any force other than the spring would have easily pushed the mirror down and the flimsy paper out of the way.
