Quick and simple.
Drill a 5/16" hole in the top (when upside down) of the bottle.
Hot glue a 5/16" nut around the hole you just drilled.
Once cool, insert your 5/16" bolt into the hole.
We refilled ours with half Mr. Clean, half water.
Step 1: Glue 5/16" Nut to the Swiffer Bottle
Drill a 5/16 hole in the bottle.
Glue (we used hot glue) nut over the hole.
Step 2: Fill
Use a funnel to fill your bottle. We used half Mr. Clean and half water.
The small funnel from a set of Walmart funnels fit perfectly.
Plug with the shortest 5/16" bolt you can find.