I dropped a couple of plugs during the install. They fall down to the bottom of the blend door enclosure. They can be difficult to see, but to my recollection there is no “lower level of hell”. You should be able to retrieve it by hand. Try recording a video with your phone placed inside the vent enclosure to get a good view of everything. You can also get a cheap endoscope from Amazon:
Depstech Single camera endoscope
Having some RTV on your fingers helps keep it from getting away from you during insertion.