"Aftermarket" Steamer Baskets
The bamboo steamer has been used throughout Asia for many centuries. They are designed to stack together over a pot or wok of boiling water, and they come in many different sizes. An exact fit is not required, but the closer, the better. If your steamer is larger than your rice cooker's inner bowl, the food at the edges that overhang your rice cooker will not steam as evenly as the food in the center. Use the lid that comes with the steamer, not your rice cooker lid, when steaming.
If you plan to use a bamboo steamer while cooking rice, keep in mind that it will likely change the performance of your rice cooker by allowing more steam to escape during cooking. You may have to adjust the amount of liquid you'll need to fully cook your rice. Some experimentation may be needed.
From the Manufacturer:
Some companies sell steamer basket accessories designed to fit their rice cookers. Be sure to compare the model numbers when selecting aftermarket accessories, as companies make rice cookers in many different sizes.
The advantage of these baskets is a tight fit and the ability to use the rice cooker's specially designed lid during cooking. This makes steaming while cooking rice much simpler, with no need to adjust liquid amounts.
These steaming baskets are designed to sit inside the steaming pot, instead of on top. They can be made of silicone or metal, and have feet to hold them above the surface of the water while steaming. Be sure to measure the inside height and width of your rice cooker's inner bowl before selecting an insert, as it will have to fit completely inside with the lid in place.
Silicone baskets are flexible, making it easier to fit one slightly too large in your rice cooker, and will not damage the non-stick coating of your inner bowl. If the insert's feet are tall enough, you can use it to steam while cooking rice, with the rice cooker's lid in place. These steamer inserts are typically smaller than either of the sit on top options above, so they will hold less food during steaming.