Earlier quoted context omitted.
What is the simple, appropriately engineered solution?
"What is the simple, appropriately engineered solution?" To hide them. The Germans are not going to spend a division of soldiers 4 years to dig up all of Copenhagen. There are innumerable places to hide them. They also could have simply been deformed i.e. markings removed, then they're just clumps of gold and evidence of nothing.
That's kind of what was done. It was a clever simple hack. I'm not sure how hard it is to prepare aqua regia (maybe they had a sufficient quantity already), but all they had to do was drop the metals in, wait, and place on a shelf with other innocuous flasks and solutions.