I use ammo cans. I just got a couple of new ones this week, too many seeds to fit the old ones. (I guess I'm sort of a seed hoarder, I can't help but collect seeds!) As long as the seals are intact and there is no rusted through spots it doesn't matter how the cans look, though I do try to buy the nicer ammo cans. The new ones have a bit of rust so I'm going to try to refinish them so the rust doesn't spread. Since I'll keep them dry anyway I shouldn't worry about how they look, they will function fine, but it might be a fun project to sand and paint them sometime when I have extra time.
I organize the seeds by type, so one ammo can has flower and tree seeds, another herbs, and vegetables seeds are broken down into summer and winter varieties. I put all seeds into zip-loc bags, with their original seed packet if it was something I bought or carefully labeled if it was collected seed. I make sure I date the bags before storage and try to replace seeds after a couple of years. Once sealed I keep the ammo cans in my basement. I've not had any problems with bugs or mold so far, so the ammo cans have been working.