Look at the label on the container first. There's a triangle with a number inside of it. That will tell you what type of plastic it is. If you go online and look up "number _ plastic" it will come up with a list of types of plastic from 1 to 7. There are only 3 (I think) types of plastic that are "food grade." I know number 5 plastic for sure is (polypropoline or something like that) - because that is what I am using for an auto top off container. If you can put food in the container, it should be reef safe. Number 7 plastic is "other" plastic and is not reef safe.
Get as thick of a container as you can, and make sure it's food grade. To baffle the sump, simply place a smaller food grade container inside of the larger sump. You then drill the smaller containers bottom and insert your filter floss and chemical filtration media etc into it. Make sure the smaller container does not rest on the bottom of the larger sump container, so that the water can flow through. The deeper the smaller container goes into the water, the more you can do with it, however. For example, you can place an in-sump skimmer if you have the right water depth. Also, plastic tote containers are much easier to install gravity fed auto top off units.
With all that said, you need to consider buying a 20g long aquarium when it's on sale at one of those big box stores for $20 - if you know what I mean. Glass and silicone is cheap - and you don't have to worry about chemicals leaching into the tank over time (even if it's food grade)
