There's no need to cover any metal feeders parts with plastic or wood to protect birds feet, tongues, or eyes. There's no evidence that birds can choke on peanut butter. Your birds will appreciate drinking water too - a bird bath or trough. In the winter, you may see flocks of birds along roadsides after the snowplows have passed. They're after the grit.
Birds have no teeth to grind their food.
The dirt, sand, pebbles, and grit they eat sits in their crop and helps grind up their food. Adding grit to your feeder is helpful year-round, but particularly in the winter and spring. Crushed eggshells do the same thing, and in the spring have an added benefit. They provide extra calcium during nesting season. In the winter, raw beef fat from the local butcher is all you need for your suet feeder.