I had a 'vert that cost me only a couple hundred bucks. Had a ratty, leaky roof. Had a similar idea as you suggested, though I didn't make mine removable.
Made it many, many years ago. Crude, but did the job...was averaging 52mpg with an XFI cam in it, in a car that was rated at and started at low-to-mid 40s.
Unfortunately, it died of cancer the winter after that. Oh well. Was a fun project.