a drop door is easy you can cut a square of 16 x 16 or 12 x 12 for smaller birds
you can use a rope or dog leashto open it
For those that have to deal with raccoons be aware that any type of pull string or pretty much any kind of pull mechanism can be figured out by a raccoon or team of raccoons in time, unless it includes a keyed or combination lock to secure it shut... Also if the door does not lock in the closed position, if a coon gains entry to the door they might be able to simply 'lift' it open by pushing up against it...