Friend's using a the larger Topeak Fuel Tank, looks a reasonable size but maybe too wide. Apidura is quite long but not very tall, both have single, central zips which I don't like.
Newer Revelate Gas tanks are good height and the zip allows opening either side. Bit pricey though.
tt? Above or below?
Because of seams you'll never get 100% waterproof. But unless rain is driving at teh zip, I doubt with a decent bag you'll get water in.