Well, here’s what I did, not for checking levels, but realized I could do it, so that was an added benefit.
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Designed for use with both the 5G / 20L and the 2.5G / 10L Scepter military water cans, the dispenser spout is ideal for filling water bottles, cups, bowls etc
www.scepter.com
I just cut the scepter spigot hose and extended it to 4feet with food grade hose from McMaster-Carr and a couple SS straight barbs. Zip ties to cinch the junctions and silicon rescue tape finished it off. Sorry, not the best pic, but you get the idea. Easy to see the water level and great to have some reach with the spigot.
A big plus is the Scepter spigot that you can’t see here. Makes fine control of the flow real easy, the lever can be swiveled to any angle and locked open too. One of our favorite mods.
You could replace your spigot with a SS pipe thread reducer of the appropriate size ending in a barb for the hose. McMaster-Carr has all sorts of fittings you cant find at HD. The Scepter is stout enough you should be able to use the threads on the reducer to tap your spigot hole.