I agree that with your credentials as they stand now you are extremely unlikely to get a federal appellate clerkship. Why is a clerkship important to you? You said that you wanted to do a federal appellate clerkship before moving on to "bigger things". Do the "bigger things" require a federal appellate clerkship? A federal appellate clerkship is a great experience, but there are few positions for which that is the only route. Law professor maybe.
As far as getting a job as a SA, I'm not sure your prospects are quite as bad as others might have suggested. I work at a 200+ atty, mid-size city firm, and we would certainly not dismiss a candidate with your credentials without any consideration. Not being on a journal will not hurt you as much in the law firm search as you might be led to believe by those at law school. I would just make sure that you have something else to point to that you did productive in lieu of being on a journal.
To get back to your specific question. The best chance you have at getting a federal appellate clerkship, like someone else said already, is going to be through connections. I think you could make those connections either by working directly for a judge or by working at a larger law firm if you seek out attorneys at that firm that did clerkships and do really great work for them. Maybe they will put in a good word for you with their judge. But as I already said, I think your prospects for fed app clerkship are slim.
That was exactly my experience applying to summer associate positions without a journal. I think out of the various screeners and callbacks I did, the journal issue only came up negatively once to my knowledge. Being able to address it confidently and explain about what I'd done with my time instead and how that had impacted my skills was, I felt, a very good moment in a lot of interviews. I'll let you know how the clerkship apps go, though -- I've definitely preemptively set my sights lower because of it.