You need to get a brush then a hoover, I've done a few of these, be aware that if you're going to block it off at the throat you would be advised to fit a vent in it too. You need to frame it in wood then plasterboard , long screws, right tools and a big bit of dexterity "should" do it. Getting a "marksman blotter" will be a total advantage there my friend . PM me if you need further advice.