need it to be about 12mm thick, get that, and it should be fine, BARE IN MIND that just silicone gel will not be strong enough, have brackets to hold the glass in place sturdy and just use the silicon to make it watertight.
should be pretty simple although the price it is just to be a plain 12x12x12 cube is near enough nothing, so its not much point.
although if your doing it for the thought of building your own tank then its an awesome idea