I personally wouldn't recommend leaving it open. I made that mistake with my first hamster and I lost her a good seven times! Hamsters are master escape artists and can manage to find their way out of even the most secure of cages so an uncovered one is just an invitation. You definitely don't want an escaped hamster, and you wouldn't believe how many members on here come seeking for advice on escapee hamsters, even more unfortunate is the number of them who actually find their hamsters. Another issue that will arise is the depth of the bedding. Syrian hamsters should have an absolute bare minimum of 8-10 inches (20-25cm) of bedding, but 12 inches (30cm) or more is really best. Having this much bedding and a wheel+other items will bring it right up to the top and your hamster could easily just crawl right out. However, creating a lid is cheap and easy! All you need is 1/4-1/2 inch hardware cloth and maybe wood (depending upon how high-tech you want to get). Good luck and make sure to share pics of your hamster when you welcome him home
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