Metal drainage grate, available off the shelf, or will need to have it custom made?
If you can not find the exact size grate that you need &quot;off the shelf&quot; then you will have to find alternative.
Custom  made OR buy a bigger one and use an angle grinder to cut it to size. Go to steel/metal supplier, you can get grating material in a variety of sizes.
This answer is as comprehensive as possible given the information provided in the question.