If you’ve lost the little eyecup for your Canon EOS 300D (like I did a few months back), you can buy a replacement from Amazon. The product link is here: Canon CUPEF Eyecup EF for EOS 300V – even though it’s not quite the same model camera, the eyepiece fits fine.

Don’t, whatever you do, try finding a definitive answer from any official Canon pages. Unless things have changed recently, you’ll spend ages looking at product charts to find that they continually reuse product names, product numbers and irritatingly close variants thereof!