I have a piece of PVC pipe with a slot on the top like this guy, basically for free!
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I don't fill mine back up like he does though. I think his way is a little complicated, and a bit messy.
I use a Ratio Rite Cup
to accurately measure the new oil that gets put back in with a regular funnel with the genny tipped back a little, and once you know the exact volume of oil that your particular genny requires, it fills it up close to the lip without overflowing. Saves a little oil, and is less messy.
Also, I lay a paper towel under the PVC drain tube right from the start of the process, because the little lip on the genny always drips a few drops when you remove the tube. Makes it all mess free, and for a lot less money. I already had the Ratio Rite for measuring oil for 2-stroke fuel mixing, but it only costs ~$6. You could use any other accurate measuring cup for this as well.
Regardless of the way that you decide to do it, make sure to warm up the engine before draining the oil, it drains the old oil faster and more completely. Also, make sure the genny is on a level surface to ascertain the proper fill level.