(If you are worried about the top shape - this is not the final mounting plate, just a bit to raise the mounting plate to the correct height)
And this is as far as I got today.
Sure, it's not perfect, but so wasn't the original. When I have time again I'll continue on it....God knows when.
In case you are wondering, I'll use it as a "counts as" RSO Panzerjager with the Unreliable trait tacked to it. Sounds about right for this scrapheap challenge.