I see someone else is an engineer. I love the idea!
Only issue I see is the mill work involved. That would be an expensive piece of billet. In my opinion, using 1/4" or 3/8" aluminum plate cut to the dimensions then have the holes drilled and tabs welded would be much more cost effective.
I do not know what a machine would charge for a one off job but it would be pricey. Much more expensive than the aluminum plate. They you have the cost of acquiring the aluminum billet will be pricey also.