It turns out that such a thing is quite easy to make. I have found one very useful for investigating the magnets in guitar pickups.
suitable. I plug the probe into a digital voltmeter to get a reading.
The data sheet is here: https://www.melexis.com/-/media/files/documents/datasheets/mlx90242-datasheet-melexis.pdf
The device runs off a 5V supply, so I used a 78L05 regulator. It might have been better to have used something a little more accurate, because the output voltage swings from just above 0 Volts, maximum north-south, through 2.5V with no field to 5V maximum south-north, and that mid-point of 2.5V end up being a few mV offset if the supply isn't accurate. Anyway, the circuit is given in the datasheet. I used this:
You need some fairly fine soldering. It helps to make sure the copper strips are really clean and shiny, use a very fine tipped soldering iron and some thin solder.
Calibration is quite straightforward, because the sensitivity is given in the data sheet, you can work out how many volts you get for a given value of milliTesla. If you wanted to calibrate "properly" you will need to put the device in a known magnetic field, perhaps generated in the Helmholtz coil that I describe elsewhere. http://m0wye.blogspot.co.uk/2018/05/building-helmholtz-coil.html
I drew a graph in Excel based on the figures in the table above.
It is worth noting that the device is not sensitive enough to use as an electronic compass. If you amplify the output you will soon find that the limiting factor is noise. The Hall effect device is quite noisy, so magnets like the Earth's magnetic field, which is of the order of 50 micro Tesla, are lost in the noise from the device.
And here's a picture of my hall effect probe being used in this way.
Hope you found it interesting.