I know this is an old thread, but dealing with a similar issue. Waterbox Stand vs. Seapora 18 x 18 x 18. The gap would make me anxious as well. Hence, why the aquarium is still empty.
In my case it is just the front.
I am going to wager had I bought the whole Eden 20 Kit (which the Seapora dimensions (450 x 450 x 450mm (17.7" × 17.7" × 17.7")) are same as the Eden 20).
I also suspect that I would have ran into a similar issue had I just purchased the Eden 20 as a complete kit. The only difference, Eden 20 has thinner glass (6mm vs 6.35mm for the Seapora) and would obviously been matched with the cabinet as one unit.
I plan to remove the doors, take the aquarium off and drill the holes for the hinges a tad higher so that the door has no choice to be flush with the bottom of the matt.
Though I hold the load bearing ability of the door in suspect. Therefore I assume, it is merely for basic support and the outward pressures would far outweigh the downward.
Rather than lose sleep (which I will), adjusting the hinge location and having the door level with the bottom of the matt will resolve the issue and afford me a nights sleep.
Gary