Pet stores and Amazon sell grated lids for aquariums when they are used as terrariums. Petco is usually the cheapest and the quickest...just go and get it. They are kept in the reptile section. But sometimes Amazon has better deals. It depends on the size you need...
[Zilla 30 X 12 inch (20 Gallon Long) tank screen] (https://www.amazon.com/Zilla-Reptile-Terrarium-Covers-24x12-inch/dp/B000QFQ300/ref=sr_1_1?crid=2W984P1UAU30D&keywords=Zilla%2BReptile%2BTerrarium%2BCovers%2BFresh%2BAir%2BScreen%2C%2B24%22%2Bx%2B12%22&qid=1660988501&sprefix=zilla%2Breptile%2Bterrarium%2Bcovers%2Bfresh%2Bair%2Bscreen%2C%2B24%2Bx%2B12%2B%2Caps%2C205&sr=8-1&th=1)
If you don't know the measurement of your tank just zoom in on the label that's on the screen in the picture.
I found a video which shows you how to make a DIY Reptile Lid made from garden wire and wood. You want 1/8 to 1/4 inch garden wire. Anything larger and the mice can fit through. But, if you don't own any of the tools or parts, then the terrarium screen would be cheaper. See video...
This video is a little more advanced...
Know that you can have someone at Home Depot cut the wood for you. Measure the perimeter of your tank top 3 times and write down the measurements. Take photos with the tape measure in place for the guy making the cuts.