IRON DETECTOR

[BENZINAS]

Is it possible someone to make me a long range iron detector with low cost and without big range and needs , i want to find some iron boxes covered with 30-40 cm of rocks in a place full of rocks , a simple detector just to show me were the boxes are when i pass next to them , detection only iron without discrimination , if anyone knows a detector-device in the market helping me do this job with rocks exept magnetometre ( too expensive ) with long range capability give me a link , opinions all accepted !