The thinking is the easy part.
- Sacrificial deck-lid you can drill and weld on?
- Turnpike-cruiser skirts that terminate in a skeg. (forms outer [diverging] fence[s] of the difusser)
- Fillet the junction of the spoiler and side plate. Cut the waste out of one or the other.
- Polymetal get's my vote, but it does leave a raw edge.
I've got a pair of Nissan NX tail lights. Free if you're in the neighborhood.
https://i.imgur.com/rdlxtuF.png
Perfect for a box cavity sidewall. Else I can always use them on a trailer