Dwayne you are off to a good start with your brass frame.
I have used this in the past and it is good stuff, but it is harder than most fillers. I have found it strong even when thin also.
This cool Rodger. I did see one drawn up like yours but they angled the back so the shelf was not as long by the rear hatch. This is not as close as to what you are doing as I thought it was.