No such thing! He'll have a terrible smelly, useless, foul tasting mess.

The anchovies have to be digested by their own digestive enzymes.

Easiest way to have garum is to go out and buy a bottle of Thai or Viet Fish Sauce. The Romans had both clear and murky versions, and so do the Thais, Vietnamese, and Filipinos.

In most cases, the murky is a byproduct of making the clear, but in the Philippines, the clear is the byproduct, thus lesser quality than Thai or Vietnamese.

They are much fussier about just what fish are used and how long it is aged under what conditions. They also have many different types to choose from. A good number of them contain intact chunks of fish or even whole fish if small) that you can crush to the texture you want. The Lao and Cambodians also have their own versions.