As Cypher says, it's fine. The only areas on the Barney I've found that are bigger than 7mm are the gaps at the top of the two front doors - they are an escape route. I wrapped cable ties there to block it. Also the top door doesn't sit flat and can have quite a large gap under it so using a couple of pegs or something to close it flush can help there.
The Barney is quite tall - not massively tall but quite a drop from the roof for a small dwarf hamster, so deep substrate helps to cushion any falls and best to put any hard toys under the shelf or hang something over any hard or pointy toys.
You can just see on this photo what I mean about the cable ties
Technically they could be chewed through but I've never found that has happened. The gap on the top door of those little bars is at least 1cm, but in the middle where the catch is, it's bigger. I've also got some cable ties on the corners as in some places the gap was a bit bigger than 7mm but not on all corners.
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