Why not? Look a lot simpilar and a lot easier to install than most of the tie down brackets on the market. Sure you've got to drill four holes but so what. A lot of the name brands require drilling to...
If you want to do belly bars, go ahead, "knock yourself out." They'll work as good as anything if your design is solid. 2" x 1/4" wall tubing is more than adequate; the commercial bars don't look like they're more than 1-1/2" x 1/8" wall. Make sure your tiedowns allow for a little flex, though, as your bars won't, and may tend to pull the mounts out of the camper.