What Bighorn said for cutting the hole.
On winter trips, I like to drain mine for the night, river rinse the pump assembly, then store it in the reservoir bucket. In the morning, I heat some water on the blaster, ad river water till it is only warm, then add my capful of bleach, in the bottom bucket.
Keeps the hand wash from being froze solid, or having cold parts break, plus, nice warm water to wash hands! Does a lot to keep people washing there hands when if it isn’t cold, and good hygiene can make the diffence between Nora Virus, or no Nora Virus!