To be fair we don't know how much traffic is in said garage. I was pretty shocked at how much dust got everywhere in our apartment parking days, even in the storage room. That's why I have covers, but they are motorcycle sized.
Zaph's suggestion of scooter covers is spot on, a huge number of scooters are used for commuting in other parts of the world, and not many of them get to live in garages. I'd imagine there are oodles to choose from.
Also not sure what you mean by "really nice fitting cover". Are you thinking of model specific car covers but for bikes? Most motorcycle covers look like the aforementioned bed sheets. Good ones are waterproof bedsheets with vents to prevent moisture build up underneath, nicer ones have heat resistant areas so you can throw them on a hot bike, great ones will be water and wind proof from all directions. But they all end up making your bike look like a blob.
Hopefully that helps more than my sarcastic post. I think you are just overthinking it. If you don't want your bike dusty, put something on top of it, otherwise, just wash it off in the spring like the rest of us. =] It's part of the ritual.
Dropped a gear... . . . . . . . . . . ...still here