Most crawlspaces have an entry like this. A large open space that allows all sorts of undesirable stuff inside--critters, moisture, mold, and more. The solution that commonly occurs is haphazard at best. Plywood covering the entrance quickly warps and molds. Leaving the entrance uncovered is an invitation for an infestation. Blocking the entrance permenantly isn't possible either.
The team at Healthy Spaces installed an Everlast Crawlspace Door, which is constructed of rigid, durable plastic that won't discolor, warp, or mold. It sits flush to the entrance, which prevents water intrusion. In addition to the Everlast door, the team installed a new access well, which will further protect the crawlspace from entryway flooding.