The simple answer to the lead filled coin problem is that you’ll make it underweight.
Or if you cast a fake of the right weight, it will be oversized.
That is only a simple answer if I carry a postal scales around with me.
Tungsten is almost identical to gold in weight per volume. Tungsten isn’t cheap, but its far less than gold.
Inserting a tungsten disc into a sliced 24oz gold coin, and then resurfacing the edge would make for a tough detection problem.
Per the lesser karat gold coins discussed here for use as smaller denominations, a whole host of metals and alloys become usable.