I had put the fractal pop air into my basket during the night and to be honest not much in the size,Something like the Raijintek Thetis is probably the smallest, but it's probably a bit too small and therefore compromised for decent hardware.
If you go slightly bigger then something like the Jonsbo D41 has a bit more useful space and several options for mounting the PSU to fit larger gpus etc
Jonsbo D41 Mesh Standard ATX PC Case – Black, Tempered Glass
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RAIJINTEK
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