Rear bumper twin exhaust to single

Hi, just wondering…as i want to go back to an oem look (NA) i would like to swap my twin exhaust for a single exhaust and …my question is; is there a way to “fill” the left exhaust cutout? I woul like to keep the bumper if possible as it is a relatively new one (due to an accident in 2014). Beside that my actual exhaust doesn’t have an EU-conformity certificate and in Belgium that becomes an issue…A couple of weeks agowhen i went with the NC to the MOT they asked for that certificate which i didn’t have, it might have brought me in trouble but the MOT-guy closed an eye and warned me for next year…I wrote IL and they supplied me the certificate so for next year i’m fine.