Buy one :P they sell them at pet stores for about $15-$20, sometimes less. If you don't want to spend the money on the clips for the lid - which, in my opinon, you should if you can - weigh down the edges with weights, heavy books, etc.
For the time being, you could take a screen off of a window and somehow weigh it down. If that doesn't fit, you could *temporarily* put cardboard (with air holes) overtop and taped down or something... I have never tried this, so someone may disagree with it.
I'd say always be prepared first. I know sometimes that doesn't always work out, but still.
Good luck!
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