Looks like you have plenty of depth to grow, which would be a great idea. I'm just wondering how it's constructed to insure the weight of the clamps, if you do fill it more, stands up to the ability of how you attached your arms. Them suckers get heavy fast, and I have seen instances where weight has pulled things off of pretty sturdy looking mounts. 

I made one at the old place that was welded up from 1/4" x 2" angle iron I got a deal on short lengths at the scrap yard, and the rack held fine, but 3 1/2" lag bolts into the frame I made of 4 x 4's pulled away. I edited this to add the 4 x4's were made from old barn beams, not that cheesy green wood most people end up with on something like that, and they were all White Oak. 

I'm not an engineer, but I call what I am thinking of a "static load" non moving. It's the force it exerts on the rack. I suppose Kenny could say if I was calling it the right thing, but I don't know if there is a formula to figure for it, but day in day out it's pulling. I do know I lost to it. Hoping you don't too.