That cap thing is easy to fix - cut a 4"x 4" piece from a sandwich bag, put it over the opening, then screw the cap on.
If you want to dilute the Phosphoric Acid, remember to ALWAYS ADD ACID - keeps down such exciting phenomenal happenings like a mini-volcanic explosion.
We use distilled water in the boiling or scalding operation because many places are plagued with minerals, such as calcium or iron in the water, both detrimental to a good finish. Rain water works well.