Solid state sensors utilizing diamond nitrogen-vacancy (NV) centers are a promising sensing platform that can provide high sensitivity and spatial resolution at high precision. Such sensors have been realized in bulky laboratory-based forms; however, practical applications demand a miniaturized, portable sensor that can function in a wide range of environmental conditions. Here, we demonstrate such a diamond NV magnetic field sensor. The sensor head fits inside a 11×7×7 cm3 3D-printed box and exhibits sub-10 nT/Hz sensitivity over a 125 Hz bandwidth. We achieve efficient fluorescence collection using an optical filter and diode in contact with the diamond, which is cut at the Brewster angle to maximize the coupling of 532 nm pump light. We discuss the potential of this flexible approach to achieve sub-nT/Hz shot noise limited sensitivity suitable for detection of a wide range of low-level magnetic fields, particularly those from electrical power systems and from biological sources.