Contamination starts when the silver nitrate on the plate gets in contact with dirt in your plate holder.
as the glass is upside down in your holder
Here I show you how to make a good working insert.
A plastice core covered with 2 very thin layers of alluminium.
The good thing of this material is that is is very stiff and light weight.
But a good matte black paint will eliminate that problem.
I did one extra thing with the plate holder.
The thin silver wires wiggled a bit so i glued the ends and this works much better.
The fitting of the silver wires is much more solid now.
Sprayed the whole inlay matte black and it's ready for use now.
This is a picture i got from Sean Mckenna a few years ago.
It shows an original wet plate holder with inserts.