Bill of materials:

1. Lock In amplifier
2. Function Generator
3. Oscilloscope (Optional)
4. 50 Ω BNC resistor (Must be a coaxial resistor)
5. BNC Cables
6. Plastic tubes Large and Small
7. 26 AWG enameled cable
8. 12 AWG cable

Tools

1. Wire stripping tools
2. Soldering ability
3. 3D printing or woodworking ability
4. LCR meter

1. Construct the driving coil: Wrap your insulated 12 AWG wire around the larger driving coil and tape it securely, we used 54 turns. The driving coil should be at least 5*longer than each of your individual pickup coils.
2. Construct the pickup coils: Wrap the enameled 12 AWG wire enough turns that the length is at least similar to the diameter, for us this was 86 turns for a length of 38 mm. Then have a gap of at least this distance between the two pickup coils and begin your second pickup coil, this one needs to have opposite chirality to the first. Scrape the enamel between the two coils and on either end of the coil. Add turns until the inductances of the two coils match.
3. Connect your BNC cables: You want the cables to be coaxial as close as you can to the coils to prevent external pickup. Cut a BNC coax cable and strip the end so only the center conductor is showing. Solder that to the end of your pickup coil. Strip the insulation from a section of the BNC where it will line up with the other end of the pickup col. Solder the outer GND conductor at that location as well. Repeat for the driving coil. Make sure your BNC is long enough to comfortably reach your equipment.
5. Connect the components like the schematic above, see image above as well. Most LIA’s have their own function generator and you can use that instead of the Oscilloscopes function generator. The oscilloscope is also optional, its purpose is to measure the current through the coil. The 50 Ω resistor allows the current to be more constant and stable, at low frequencies the impedance of the driving coil is very small and the current will likely be limited by the current output of the function generator and will not be able to supply the voltage it is set to. 
6. Apply settings for you instruments:
   1. LIA
      1. Single ended input and output
      2. Reference to external and select the port you connected to function gen
      3. Typical Low pass is BW 3 dB of 100 Hz
   2. Function generator
      1. Max voltage that does not exceed power limits of your resistor
      2. Impedance to high Z
      3. For calibration I used 5 KHz but checked the response at multiple frequencies.
7. Centering Coil: 3D print at least 4 of the spacers included in the files or print your own to match the dimensions of your tube. While applying the oscillating voltage to your driving coil at 5 KHz (higher frequency stronger signal but if too high parasitics will add error), move the center coil back and forth until the in phase component of the signal is as close to zero as possible.
8. Verification: With the pickup coil fixed, slide a sample through the center of the device taking measurements along the way. It should look like the image below. If one of the humps is greater than the other you need to adjust your pickup coils to have the same inductance. 3D printing a sample holder to get it centered in the pickup will help your measurements especially at higher frequencies.
10. Record data: Place your sample at the location that maximizes the signal strength and record the signal magnitude and phase. Recording the STD Deviation of these values can also give you a sense for the error.