It is kind of hard to see in the pics, but the braces on two of the corners stick up above the top. The second pic uses two dog crate dividers...plenty strong, but maybe not the best weight distribution, and I could see the rats in the top tub still, etc.

3rd pic is the most promising I think. It raises the reservoir for better flow, the little stand happens to sit perfectly between the edges, so long as they will hold the weight. The only major problem with this is that the top of the little stand spins, but a simple screw through it, or just removing it, would fix that.

The last one is probably the most stable...I would remove the legs so as not to spin. This is my second favorite. I can't really see any problems with it.

Let me know what you think would be best of the above, or none of them.