This squirrel resistant bird feeder uses a 20oz soda bottle as a hopper. The perches are supported by four springs, so that when a heavy creature like a squirrel climbs on it, the food doors close. By varying the strength of the springs, the sensitivity can be set.

A few small springs and two short M3 screws are needed for assembly of the actual feeder portion. Multiple shorter springs can be combined using the optional spring spacers. Three more M3 screws mount the roof. The three bottom pieces can be printed without supports, the roof piece unfortunately does require them. I would use some type of filament that doesn't degrade quickly under UV light and has some thermal resistance as the feeder is used outdoors.

I use a small clip to make it easy to take down to clean or refill. I would hang it with monofilament fishing line to make the squirrels' lives more difficult. While the spring system works well, a determined squirrel could just chew through the feeder, so it is best to have multiple lines of defense.

I fill this feeder with the help of my screw on soda bottle funnel.

Thanks to steffanders for the bottle threads.