I like going with the vegetable idea, because back in those times that is what I usually tried out. Carrots are good, because they are long and for that reason don't seem to need a flared base to remain safe. Don't want anything going all the way up there do we? You are going to want to buy a real one along the lines though, because no imitation will do a good enough job as a real butt plug. For me it has gone from carrots (wasn't that common), trainer kit (three different sizes), and then a vibrating butt plug (one being wirelessly controlled and the other wired).
Oh, and it doesn't seem to matter even if I used a carrot, lube is important to use. If you don't use any, I would personally advise against it. I'm sure there are reasons behind it.