Ditto on Expanding foam. I find it a great way to seal out the mice..
and it is easilly removable if needed. The only downside is it is
messy, and looks cheap when done, but it is your basement floor so who
cares if it can get the job done.

Just cut a piece of sheet metal so it is just small enough to rest in
the hole, with maybe a half inch clearance on every side, temporarily
duct tape it in place, and fill around it's edge with the foam. the
foam will expand and lock metal in place, It will look messy, but it
will seal that baby up tightly, and keep mice out. You could just
use a plywood board, but they could chew through it.