Copper chests are a good place to start. But since they drop from trash mobs, that complicates the experiment. You'll have to make sure you land X stacks of Snatch average on each mob (and a minimum of X stacks on the boss). My initial power estimate assumed 5 stacks. We can lower that number, but then you'll have to collect more chests (e.g., 300 for 3 stacks, 700 for 2 stacks). Meanwhile, the control group will do all runs without Snatch. For each group, record the number of chests of each rarity that drop in each run (just record the final tally the game gives you before you hit the next button) until a total of 100 chests of the rarity of interest are collected (more if less than 5 stacks of Snatch per mob). We can then do a t-test on the series of drops per run in each group.
As long as each person uses the same methodology, numbers can be pooled to achieve these targets.