Why did it take me 3 years to think of this?
It’s not like it’s a terribly complex device, especially when you consider that I soldered up something fairly similar for a sprinkler last year.
But hey, I thought of it now, and here it is.
No more hoses slipping out of the top of the barrel and spraying water all over the place. Just hook it over the lip of the barrel, turn on the water and forget about it until it’s all full.