Suspended nanochannel resonators (SNRs) are promising devices able to characterize mass down to the attogram scale, thus being able to detect nanoparticles or biomolecules. In this paper, we present a flexible fabrication process for SNRs based on a sacrificial layer approach that allows to easily tailor the dimensions of the nanochannel by changing the thickness of the sacrificial layer or its patterning during the lithographic step. The resonance properties of the fabricated SNR are investigated in terms of resonance frequency and frequency stability (Allan deviation). Liquids of different densities are injected in the device and, from the shift of the resonance peaks, the mass responsivity of the resonators is assessed to be up to 3.90 mHz/ag. To the best of our knowledge, the devices here presented are the first example of suspended nanochannel resonators with a channel height as low as 50 nm fabricated with a top-down approach.