What do you guys think is the best size hole to drill in the cap for the feeder?
Couldn't tell ya what size hole to drill, but have seen some made with the "feed cups" taken out of those cheap store bought wild bird feeders and installed into the side of the pipe. They have the part built onto them that holds the feed back in order to stop the constant flow.
If you wanted to drill holes, you could start with a smaller hole and just keep drilling it bigger until you get it the way you want.