I'd secure the pieces together, through the factory holes, using several 3/16" clecos, then ream each hole to .26", while securely flanked by clecos. To yield a wrinkle-free appearance.
Are you using a sealant between the two? My factory ceiling skins were glued & screwed, originally. Clecos will quickly align & hold it in place while installing either rivets or screws.
Jay-Cee sells clecos on that same site.