Since you are not interested in depth, you don't need a large (e.g. 6'x6') coil, but can make a wide and narrow coil like 6'x1'. This should still give you about a 1' depth, be more sensitive to smaller targets (than a 6'x6' coil), and better match the inductance of the normal mono coil.
I have designed and built metal detector electronics for my own use but never made a large coil detector. The first issue is whether to use a VLF continuous wave, or Pulse Induction (PI) design. Most systems like this use a PI design because the coil is just a simple loop.(mono coil). VLF systems that drive the coil continuously, need a design where the net magnetic flux into the receive part of the coil is carefully nulled to zero within about one part per million by careful mechanical design. This usually takes the form of a double D where the loop is a figure 8 with overlapping s