I haven't heard of a non-3D background going on the inside either, and it's probably a bad choice in marine anyway. Among other things, depending on what it's made out of, grazing inverts would rasp it away over time (and some inverts can strip the paint off of other ornaments pretty fast).
Can you scrape off what's left of the old background and redo a new one on the outside? You'll have a lot less hassle in the long-run that way. If you really want a solid color background too, you'd likely have to scrape it regularly to keep coralline algae off, which would certainly be easier when scraping it off of the glass/acrylic directly.