As a kid, I had a solar power experimental kit with 150 experiments, designed by Avi Sochaczevski and Peretz Mahler, published by Tree of Knowledge in 1989. It came with an amazing DIY galvanometer which I tried to recreate here.

Warning. This galvanometer is designed for very low voltages of 1.5 V!

Building instructions 

When you turn the galvanometer body over, you see the space for the wire coil. I used a coated 0.3mm wire, and wrapped it around probably a 100 times or so. That's significantly more than the 15cm long wire as suggested the original instruction manual. 

In order for the galvaonmeter to function, you also need a pointer with a magnet holder which I have not designed yet. 

References

• https://www.pololu.com/product/1674 seems to be the original product page, you can still download the manual there, the galvanometer is described on page 42.