I had a cabinet like this, there are a few options. 1. If you have the proper tools and can do so safely, you can remove the front from the drawer that fits in the bottom. This will maintain the opening at the top while filling in the bottom portion. 2. If not, you can use a piece of sheet metal and cut it to size like I did here. Once you fill it, it seals enough without fastening it. 3. If you're putting two next to each other, you can put the open end flush with another cabinet and fasten them together.