I don't remember where I came across this idea, maybe it stems from my childhood fun. Anyway, an idea for a butt plug is using playdough and molding one. This way it's the size you want, and will have a flared base. Obviously I'd use a condom with it, but you can get the stuff that will dry out and harden. I'd think you could coat it with some kinda rubber agent, rubber cement even? Still, where the condom, but that might give it more of a rubbery feel. The playdough can be purchsed rather cheaply, and it's one of those items that no one will look twice if you get some, and some rubber cement.

How this stems from my childhood, I once found a condom in my parents room (yes i was snooping, and my mom had one of the lady bug vibrators, too bad I had no idea what it was at the time), and I knew what the condom was, what it was for, so I filled it with playdough to make my own little penis. It's that penis that took my virginity

So there is my 2 cents and my story, hope it helps. If anyone gives this a try let me know. I might go out and experiment today. Might be a little tricky with the narrowed before the flare, try sticking a popsicle stick for some added security.