Thanks for the suggestions so far. Here is a pic of the hole. Also, my memory was way off on the size of the hole.
Spoiler for size:
The hole is 91 inches deep x 11" x 14" and is 10" from the side of the house.
The underground sprinkler cover idea would have been great had the hole not been situated smack dab in the middle of concrete. Are there any other hatch options that would work for this scenario but made of metal. That way it might work while still allowing the necessary ventilation and whatnot. I don't think I could screw it into the wood directly. That stuff is just as rotted as the plywood cover on it before.
I've temporarily taken a concrete stepping stone from my back yard and placed it where the rotting wood cover used to be. I wouldn't want a kid to step on that plywood and have it snap under their weight.