The objective is to stop rabbits from wandering inside my backyard. They do it before even moving the seedlings outdoors, and survival of anything (other than spiny roses and raspberries) would be a lost cause.
If not concrete, how might I close this hole?
You won't keep rabbits out with anything short of a full-length barricade well below grade under the entire fence. It's not a practical goal (nor do I understand the human inclination to completely detach ourselves from much of nature).
Put your plants in a dedicated enclosure or on an elevated platform.
