So my garage floor had two very intentional holes that I put into an access panel; one as a finger pull, the other as a cable run. Mice thought these were really great for getting into my garage too. So I had one of my guys route two circles a little wider than each hole  and two matching width rectangles. Then I drilled and counter sunk pass-through holes in the circles, but tapped holes in the rectangles to accept the thread on the bolts. Carefully drop this oddly gaped sandwich through the hole, recenter the disk and then tighten, which creates a clamp at both ends of the rectangle. Holes plugged, no more mice. Hooray!