Would The Last Starfighter count as a Star Wars knockoff? I’ve heard some people say it is, but I don’t think it’s quite derivative enough to be called one.
I guess it depends how you define rip-off. I’d say, not really, that falls more into general “inspired by,” but then again so does Battle Beyond the Stars.
Roger Corman got a lot of mileage out of recycling the FX and the score. It would be a interesting experiment to make a low budget space movie using that footage again and make it look like some lost film from the 80’s.