As Angus said, it's structural. Another thing to consider is that there should be seat belt mounts in that panel, so you can't just whack a MDF panel in there and bolt seat belts to it. Well you
could but you would make you a complete goose
As suggested I think the best way to go is cut the shelf from a wreck and weld it in. Then you can go ahead and make a MDF panel to fit on top of that.
And I would suggest that you make your MDF panel BEFORE you put your rear screen back in, as it's about eleventy billion times easier to do all the marking & installation without the screen in there
