Decent grade methanol used with lens tissues will not leave streaks. It's very commonly used in the industry for this reason. Simple, cheap, highly effective. To reach down into a recess to clean an optic, fold the lens tissue as many times as necessary to get it to an appropriate size and use plastic forceps to hold it. Place a drop or two of methanol on the folded tissue and gently wipe. When folding the tissue, don't touch it anywhere near the part that will be used for cleaning. Preferably fold it while wearing gloves so as not to transfer oil and salts from you skin to the tissue.