Those would work fine too. Take a look at the "XL" series of Precision Marine bullet skimmers (
http://www.precisionmarine.com/html/xl.html ) or, say, even Titus's skimmers (
http://www.canreef.com/store/skimmer.php )
I would venture that it's a little easier to construct using the box at the bottom, because then you need only worry about bulkheads through a flat surface instead of a curved surface.
Another thing to consider though is that beckett skimmers tend to burp from time to time. This mixing box might help address that. Basically, you want the foam to be level. If there's any waves or sloshing, it may be hard to get a predictable setting.
But, those are just theoretical. Basically, if you can get foam, you can get skimming. There are no real hard and fast rules per se.