The issue is onions have chemicals in their cells that merge to form the compounds that we're used to. First, you need to minimize the damage you do to the onion, as you're otherwise releasing the chemicals too early. This means for the most part using a very sharp knife.
Depending on what you're going to be making with the onions, you can put them in the freezer instead of the fridge (but that's generally not needed for this short of a time if you use one of the other recommendations below); if the onions are going to be cooked, you likely won't notice the difference in texture. The cold helps to slow down the chemical reaction.
And your last option is to change your onions -- consider 'sweet' varieties of onions, or red onions, that aren't going to have as strong of an onion flavor, but also won't outgas as much as a result.
And um ... use a glass, corningware or similar container, with a tight fitting lid. I find the smell seems to permeate through plastic over time. (it might not be an issue for only 6-8 hours, but you never know).
And the last option -- don't finish chopping them. Halve them, peel them, and store 'em cut-side down in the fridge, so you've already reduced maybe 1/3 the time to chop onions (assuming you have sharp knives and good knife skills), then just finish 'em at the last minute. Maybe recruit a second person to help.