Opps.... I should have said
Sodium Persulphate not Sodium Phospate. Maybe you will have better luck finding the right stuff. The product I use in made by MG chemicals.
Use Zip Lock Freezer bags. They are sturdier then regular zip locks. I would do the etching over, or in, a sink in case of a leak. Would not do this with any etchant other then
Sodium Persulphate. It is very tame compared to the acids or Feric Chloride. I try to avoid getting it on my hands but do not wear gloves. Just wash them when done. Do not take my word for it, check the material data safety sheet.
I etch with Sodium Persulphate in a glass cake pan. I heat the water in the microwave then put the solution in a pan under a halogen work light to keep it hot during etching. I use just enough water to cover the board. The used up etchant goes into a butter tub and the water evaporates leaving blue crytals.