Wow.
That looks very nice, but I must agree with Liquid... not square, and you are off level.
If it was a 10 or 20g I'd say go with foam and, or a shim, but I think you should try to shave down the high corners. Get your self a good wood file for $12.
As a man that has had 55g of water on his floor due to a DIY stand, to a man that COULD have 150g on his floor, let me say this: Take pics of that rug now, cuz it could get a whole lot worse.