I don't know what size your wire "cart" top is, but for a small wire shelf rack that I put in the kitchen, I found floor tiles that were EXACTLY the right size to drop down into the top. They fit perfectly front to back and there's a couple inches left at the sides. They are very attractive and only cost me two or three dollars for both.
Tiles are all just slightly different in size, I bought these tiles at the same time I bought the rack so I could fit them in until I found two that fit in snuggly, not too loose and not too tight.
Alternatively you could take just the top shelf with you to the hardware store to find the perfect combination of tiles. If I had a tile cutter or nippers, I could easily have found a 1 piece rectangular tile to fit in there, just nipping off the corners to clear the round posts. I plan on doing something similar for my other sets of wire shelves eventually, but I needed this right away so I could use that little shelf to set my blenders on.
Yes I said "blenders". My son's, and mine. His is better for some tasks and mine is better for others so we ended up keeping both.