Can a coin detector be made....

Dig up all the foil signals! If you have a gold nugget at home, test it on the detector. If you don't, use a chocolate wrapper, and dig all those signals!

Your other option is to buy a detector dedicated to gold prospecting. The xterra705 or the gold monster are some of the cheaper options. However, they don't perform as well as Minelab's GP detectors or Garrett's ATX (especially in heavily mineralised ground)

What detector do you run?

Higher frequency detectors ID small nuggets better.
Marvin