If you put more than one piece of copper the pieces tend to bend in the center so I wanted to be able to put 5 pieces of round copper to etch so it needs support.
I found the easiest and lightest pieces are those foam nail files you can get two for a dollar at the dollar store.
They have foam centers and are just long enough to fit into the glass container - if not I could cut very easily
I tape my copper pieces as usual then place the file on top and put another piece of tape. Voila - lightweight and supports the pieces and lets them sit in a straight line on the surface of the ferric chloride
so I can make 30 of the etched pieces at a time
Another tip - as you put your taped pieces in just write the time you put it in so you can gauge when you should check it - sometimes I've forgotten when I put in - although I just check the piece visually