How to reinforce existing shelf

[Orangutran]

Hi Everyone,
First post here. I'm in a situation where I can only place a fish tank in an existing built-in shelf that came with the house. See diagram. Anything white is how the shelf is now, and anything orange is what I'll be adding. Note that the bottom half (inside cabinet doors) will be re-inforced with 2x4s, and lets assume that it'll be solid down there. My only concern is that extra gap between the bottom, and where I'll be placing the tank.

Tank will be a standard 29g, so after I add water, etc, it should be less than 300lbs.

Let me know your professional opinions. I know we all like to overbuild our stands, so please be mindful that I do not have the luxury of adding 2x4s all over the shelf where the tank will be sitting.

Thanks in advance for your input!

 

[Reefing Madness]

 

[Orangutran]

Thanks for the free bump!

I know it's hard to say if my plan is good or not, but don't worry, I won't blame anyone if the tank ends up on the floor, because my wife would kill me first!!

Just today, I spoke to a friend who worked in the lumber industry, and he said 3 pieces of plywood with the 3 points of support will definitely hold the weight. He said I should be more concerned about the bottom half, so I'll definitely over build (add supports) that part to take the weight, and keep it from shearing.