Go get a small 1 or 2 gallon garden sprayer and put a 1/4" ball valve on the hose end.
I put a 2" pipe nipple and elbow on that so it hangs in the hole.
When it starts to drip at the fill hole , turn off the valve.
Can find one at the dump as it is usually tossed cuz the sprayer is broke/plugged. But you don't need that part of it anyway.....