For my 75 gal stand project I took a hollow fully flat "door' assemblty (very cheap at homedepot), I uniformly spaced three added three tubular aluminum rails (3/4" by 3/4" and 4' long) on the inside of this hollow internal 'box' structure (the rails ran along the length of each side and one ran down the middle) and this formed the top of my 'cabinet' for the aquarium. The end of 'door' that I opened had this piece reglued back into place. The top looks professional and better still, easily supported the full weight of the aquarium in a uniform manner.

You could use floor fill to level the top of your stand and then place a similar reinforced support so even if the top warps some, the metal tubular rails will carry the load. Do not depend on just filler - water can damage it and it is brittle (i.e. breaks and cracks in time under load.)