Securing wine cellar vents
You will need to form up and pour/trowel a concrete frame to hold a steel grate. You should be able to find a local fabricator/welder who can make them custom to your size needs, or an internet search will reveal fabricators who will make and ship them to you.

 

Another option would be to build a wood version yourself. I believe you could build a "slatted cover" that would be plenty strong, but it would not provide as much ventilation as a steel grate and would require some frequency of maintenance. It would be significantly easier and cheaper, however.