Are you sure you need a rear spoiler ? Is it for looks or handling ?
For looks, chacun a son gout... but for handling, where ? On the street its useless. The vintage race groups probably wont allow it. If your interest is in track days, then you must experiment to determine downforce vs. drag.
The simplest and probably the most effective would be to make a spoiler out of either Lexan or aluminum sheeting, bolted to the rear edge of the hatch lip. Vertically cut slots should be used to raise or lower the spoiler as needed. Measuring results will be the biggest chore. Have fun !