First Principal.....
We did use a portable unit on two occasions. It was a unit designed by a Marine Company in Florida. There was a "waterproof" model and a standard model. It consisted of two lengths of what I will call "Plastic Drainage Pipe" about 2 feet (600 mm) long which joined together to form a long tube carried by a Shoulder strap. Power was supplied by rechargeable batteries stored in the tube and with a Control Panel at one end with Headphone socket and Volume and Setting controls. The unit had to be preset to the Earth's Magnetic field according to a diagram. The cost I recall was about $1500.00.
The carriage method in the Jungle was a bit of a problem, weaving in and out between foliage meant it wanted to swing also and the Instructions demanded a steady path for the unit. It also meant that bushknives and spades had to be well away from it while in use. One other problem was that due to the batteries being low charge rate the charge had to be 12 Volts and under 0.5 amps to avoid cooking the batteries so that required a separate transformer to be along as well.....
It was also the reason why I started to accumulate about four or five generators down there. On the early trips we had relied totally in batteries, now I needed a GenSet to charge the Magnetometer batteries.
I found that once I started taking Generators in, it is amazing how the team members managed to carry all manner of those rechargeable electric gadgets with them on the next trip and a queue formed at the Generator....!