I plugged the vent and filled the barrel with dishwater.
I do that often also
Cayugad.
You don't mention what you use to fill the barrel. I use 8 Oz Plastic Cylinder Bottles with Flip Top Pour Spouts as shown in the link below. I got mine from a local craft store.
I fill one with a dish soap & water mixture and one with plain water. They make it real easy to fill the barrel.
https://www.amazon.com/Plastic-Cylin.../dp/B008IS1802