There are some things that are not meant to work very well. Unless the stand is open on the back and the tank is added before you slide the stand to the wall, it will get messy to do it. Doable but messy. Not to kick a dead horse but that stand would not suit me for a large tank. With the top design, it will be very difficult to get a 125 lifted off the stand. Unless they were designing for earthquakes zones, the top lip is excessive.
How to get the tank into the bottom? It looks like cutting the front support out, slide in the tank and repair will be one way. Another way might be to remove the front center support, add some support across the opening and then rebuild the front.
Either way? Very messy!