Plexi/acrylic/lexan backed box
 

My brother in law is planning to add a sub to his E325 the box is obviously going in the trunk with the sub facing into the front cabin (mid rear seat has a "ski slot") didn't measure what size sub can fit in the slot yet but that's besides the point, he wants to do a plastic rear for visibility into the box with a custom BMW welded on the rear is there a way to bond a whole panel like this to mdf that is as good as framing complete mdf box?

I don't follow. It sounds like the clear panel is going to be the entire back wall of the box? If so, drill and screw. I like to use a T-nut since button head wood screws are a tough find but you can get trim washers for use with countersinking screws. Use weatherstripping as a gasket.