For the “quick” part, I’d rely on the notion of “small,” to which you could add either “vite fait” or “en passant,” (or neither, and maybe even both):
“Un petit merci vite fait”. (from LWS.fr)
“Un petit merci, en passant” (from blogues.lapresse.ca)
As to how to present/preface the sentiment, you could use:
“Juste un petitepetit merci … pour” (from jeuxvideo.fr);
“Voilà un petit merci … pour” (from jeuxvideo.com [down in the comment]);
or (unless you find it too close to the usual suspects)“Je voulais juste te dire un petit merci … pour” (from cuk.ch)