I just got my first clam a few weeks ago (so I'm by no means an expert), but to spot feed I cut a 2L soda in half and then glued a piece of 3/4 inch diameter pvc pipe into the hole where the cap goes. It's a perfect fit, and it's much easier to spot feed the clam, just place the bottle around the clam and pour the phyto in through the pvc.
-Jonny