Honestly, I find it highly suspect that you only print the first 7 characters of the hash. I wrote a script that finds collisions within 8 seconds on a normal desktop. Given a minute between spins, this gives you plenty of opportunity to pick a number that favors the house. To immediately address this, and if you are concerned about the full hash taking up too much "real estate", use a tooltip so that when we hover, we see the full hash. Then, and only then, will this be "provably fair". Because right now, this is "provably fair theatre".