Since the MagArrow Magnetometer only weights 1 kg it will little impact on your 3-5 kg payload drone flight time.
Our MagArrow uses a MFAM sensor to provide high quality magnetic data with noise/sensitivity range of only 5 nT. Most people have suspended the system with non-magnetic cables to remove the magnetometer from the magnetic influence of the drone itself. Some customers have decided to build a fixed frame to mount the system but generally it isn’t large enough to remove the magnetometer from the drone’s magnetic effects so there will need to be advanced filtering to compensate the magnetic field readings. Either option is acceptable.