For small spaces like that, my brother carpenter has always made them easy to remove (ie don't leave them down there to get in way, when you need to get into crawl space). Since you don't have any freeze issues in Los Angeles, CA then I'd recommend digging out several inches of dirt and putting down landscape fabric then gravel or rock.
Depending on how secure you want to make it, you could install a "bottom" made of angle iron with some corrugated steel attached going down into ground about a foot so critters couldn't dig under. With a square opening, you could make door that is tapered (again steel frame) so when it's pushed in, it would seal off the edges. A couple rods on both sides that slid into holes like a hinge (think wrought iron gate) could be secured on each side with locks by drilling hole through the rods. Middle part of "door" could be wrought-iron style bars with screen on backside to keep out insects.
That's my 2 cent idea !
I'm thinking a custom "door" by a wrought iron fence company would be great. Same fence company could make you a "lid" for the opening so that animals or people wouldn't fall down into that hole ; it would have hinge up on house to get it out of the way when needed.