why do you want a drop noseband? If horse tends to open his mouth I'd worry bit was uncomfortable for him so I'd look at a different bit, rather than making him close his mouth around a bit whcih migth not fit his mouth and might be hurting him.
My instructor recommended a drop noseband, instead of a flash as it looks a bit nicer and you get a bit better leverage on the mouth it doesn't have to be really tight.
You can't tell a horse to close its mouth if it wants to be a bit naughty and you're training it! You know sometimes they are comfy, maybe in the arena, but start going faster, XC whatever they can open wide and tank it! So you have to persuade them for a while!
Sorry I haven't got a nice one close to, I'll try to put one on here, a bit further away. But really, make sure:-
It's on the nose bone at the front, just, so as not to affect the airway, by squashing the nostrils.
It's below the bit, around the lips, snugly up to the bit.
Make sure you can just fit two or three fingers in when done up.
I'm worried mine is a bit wide across the front, the rings on each side actually go over the bit when I do it up to the correct 'tightness'; although this does improve a bit when you take a contact... i will have to take a photo!
Is it a modern one that you have? It can be hard to find a drop that is the correct size, especially new ones. They often have a top part that is too long as you describe.