First, I'll answer your question, what am I using for a stand. I am using a mobile kitchen island for the stand. It's solid wood with a decorative wooden top. It had casters on it, and I removed them, and one side of it folds down. The tank fits on it with room to spare with the side folded down, so I use the folding table for when I check the water parameters, it makes a nice surface to put everything on.
Now, having said that, I would urge you away from the Flex. I bought it for my daughter about a year ago, and it has a lot of quirks that just make it a really undesirable tank. The biggest issue with it is that it absolutely fails as an AIO. The rear AIO housing is not sealed properly all the way around in critical areas which allows water to bypass the filtration entirely. Also, the lights that come with the tank are only so-so and almost everyone I know of has upgraded them to at least a pair of AI Primes. Other issues include having a limited amount of aftermarket "upgrades" that work with the tank and its AIO dimensions. Your money is better spent elsewhere... A Fiji AIO Tank would be a much better route and about the same cost, or, you can get yourself a 40-breeder and get one of Fiji's drop in AIO kits for it which is a far more economical approach allowing you to save a bunch of money and buy other, better, more critical equipment.
That's just my $0.02