Find yourself a flexible brush made just for that purpose. Some LFSs will carry such a thing. You could also try cleaners meant for waterpipes and hookahs.
I was lucky enough to have one included with my Marineland Emperor filter.
Tie a piece of string/fishing line/whatever to a small rag/sponge/whatever. Tie a small nut or weight to the other end and drop it through. Pull the rag through and your done.