I have my NAS on a shelf in a mini-ITX case, but it only fits two 3.5" HDDs internally (as well as an SSD, but full-size HDDs are what matter for bulk data storage, the more the better)
Also, it takes a normal full-size ATX PSU because I was fed up a previous case that only had room for its own custom PSU, which kept failing under load. But I note there are now standardised small sizes like TFX12V and LFX12V, are there any efficient and reliable PSUs in these form factors?