Based on two sources of information about that protest I've concluded that it is at least 99% certain that the decorations landed on or near steps of the Capitol, with none going into any kind of container.
One source is the film Going Upriver: the long war of John Kerry, which I saw Oct. 1, 2004. The other source is the book Home to War: the history of the Vietnam Veterans Movement. The film shows around five minutes of that protest, ending with a shot showing a pile of documents and medals on the bottom of some steps and upon pavement near the lowest step. It formed a mass around five feet square, and I could see no trash container or any other kid of container anywhere near the pile.
The book Home to War has a description about something that happened after the last medals were thrown that has me convinced that using a trash bin would have been very repulsive to the men throwing the medals. The book says that after the last man threw his medals, two separate times some souvenier-seekers climbed the fence and tried to take some of the decorations. However, each time this happend veterans seeing it became enraged, Home to War says, climbing over the same fence and chasing the souvenier-hunters away.
The book explains that many of the the medal-throwers regarded the souvenier-seekers as descrating the dead: many of the medals were those of close friends who had died in Vietnam.
Going Upriver showed several instances of men naming the friends whose medals they were throwing. These occurances suggest that what O'Neill said (shown below) on page 143 about "everyone" assuming that Kerry threw away his medals is false. Surely everyone there assumed that the what Kerry threw over the fence could have been medals of one or more of his dead friends.
The question arises: what motive could O'Neill have had to make this lie? I think there were several: (1) to make Kerry and the other protesters appear akin to extremist pacifists -- to seem as if they were rejecting all war, (2) to make the protest seem something like flag burning -- to make it appear as if they were getting rid of the medals entirely.
into a bin that had been marked "trash".
Kerry, too, approached the microphone, said his piece, and three away a handfull of what everybody assumed were his medals.