Would this work?

[privet01]

I guess my thinking was that if the container was full of substrate (and possibly gravel in the bottom) that it wouldn't float in just a few inches of water...?

It might not float depending on how much weight is in the container and how much water surrounds it. You can always try it out and see the step back and punt if it doesn't work. If you don't try you'll never know for sure.

How could I prevent the built up gravel from eventually washing out without some sort of barrier?

I just used regular wash gravel from a construction project. I sieved it to be nothing smaller than a 1/2 inch. The irregular shape of the gravel and differing sizes allow it to hold it's shape very well even though the dry side of the gravel is as much as three inches higher than gravel on the water side. Occasionally a few rocks get kicked around but those are easy to deal with at cleaning time. Sand would probably help hold it in place too, but I don't use any as that's just more weight. And you could always use aquarium cement or a safe silicone adhesive.

Might be an entirely different story though if you use a very smooth and very round stone. I suppose the gravel might shift if the viv gets bumped into quite a bit or you have a strong vibration from a 10 foot sub-woofer placed behind it. <<grin>>