Huh. Mine is marked on the outside already. Have you tried suspending a flashligt on the inside?
Maybe make you a dipstick that you can stick down in the barell after adding each 5 gallons of water and make a mark on the stick and label it as you go. When you want to know how much water you have, just insert the stick in to the bottom and you should be able to get a reading off of it when you take it out. Make it out of some sort of material (like a long wooden dowel rod) that will show the contrast between wet and dry easily.
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