I think I've decided to put a fill hole on the top cap of the caps I'm designing for my personal tanks. That would be extremely handy, I think.
You know, I thought the exact same thing til I got the prive tank. The first time I tried to refill, I lost the screw and spend 1/2 an hour crawling around the floor in a panic looking for it. Heck with that, never used it again. There's another tank... forgot which one, that had a press in metal button fill hole. I don't have that tank, but the reviewers said it was a great feature.
But... I'm pretty sure I'd lose that too if it wasn't attached the tank. The problem is, the tanks themselves are just too small, and the fill hole cap would be proportionately smaller. If it's small, and removeable, it can get lost. If it can get lost, I will lose it. If I can lose it, I don't like it, and won't use it. If I don't use it, I don't even want a tank with that feature, because I will somehow knock it off the tank one day, or break it, and then can't use the tank itself. Not sure how others see it, but that was my thought after I got a carto tank that had a fill hole...