Making "long(er)-life" homemade mayonnaise
I make a pretty large quantity of homemade mayo and have never had a problem keeping it longer than that -- Good Eats uses a week for their recipe and for me it lives a month, easy, with no detectable reduction in quality. There really is enough acid and salt to deter most bugs if you like it strongly flavored (and let's be honest, mayo should be strongly flavored because it's a sauce made of fat and you might want to minimize the quantity applied). I'd also take the advice of stilltasty.com that while color, flavor, or texture may change it's generally still safe to eat refrigerated mayo even after it's notional "use-by" date.

In short, I strongly advise more salt and acid if you're worried (and also because it's tasty), but I also strongly advise a lack of worry.