I recently made a simple box cake, the one where you just add water and oil to the mix, but once I ate it I thought that it was way too sweet. I looked up how to temper this sweetness, but the best I could find was a suggestion to add lemon rind or - I assume - lemon essence (to a vanilla mix cake), to try and balance the sweetness. Is there a better way to do this? The best I could think of is to add flour, but the website also said not to do that as it would make the cake ingredients disproportionate. What could I do instead?
There's not really anything easy you can do. Even adding lemon zest or essence isn't really going to help; it'll just give you a lemony but still sweet cake. I suppose theoretically you could add sour things, since that does tend to balance out sweetness, but it's tricky since acid will interact with the leavening. A sour icing, or a sour sauce to serve it with, might be a better bet in that vein.
So unfortunately, you'd be best off shopping around for cake mixes that are more the level of sweetness you want. You might be able to use the nutrition facts to help get a sense of how sweet they are before you buy them.
You need an alternative to regular boxed cake mixes, rather than trying to compensate for them.
Rather than going the "from scratch" route, you could buy a box of cake flour at your store. Some have simple cake recipes on the box. You do have to add additional ingredients, but since the sugar is added, you can adjust it however you like.
If the product has a website you might find some recipes there too. For example: http://www.pillsburybaking.com/products/softasilk-764 has some recipes.
Replace some of the cake mix with flour. It's basically just cake mix but without flavour, so it could replace some of the sweetness. It's what I do at home and tastes perfectly fine.
The amount you replace really depends how sweet you want the cake to be. I replace half of the cake mix so that I can use the other half next time.