OK, OK! Just trying to be helpful!
I've actually thought about the following method, but never tried it.
1) Get a few pieces of plywood and stack them to the height you want the plug's "stem".
2) Drill holes of the desired diameter into all the plywood (nail or glue them together)
3) If you want a wider "mushroom top" area, then take another piece of plywood and mark the centers of all the holes you drilled in the plywood stack. Drill a larger diameter hole in that plywood (or another stack, depending on the thickness you want the top)
4) nail/glue the piece with the larger holes onto the first stack.
5) Place a piece of plywood under the mold.
Now you have a wooden mold that you can fill with aragocrete/cement with that will produce mushroom shaped plugs. Since the bottom piece is not attached, you can just invert the mold and push the plugs out with your fingers!
Now, I understand that wood and wet cement probably don't go well together, but maybe using some urethane or some other type of coating would make the mold more durable... or else maybe it's so cheap, you don't really care if the mold is ruined. I haven't tried it yet so I don't know how sticky the aragocrete would be on the molds, though I do know that in building construction, plywood forms are often used for pouring concrete...though only the faces are exposed to the concrete, not the EDGE of the plywood, as it would be in that case. (but you can use non-plywood, of course. Like if you have some thick acrylic sitting around, and the ability to drill it.
I'm talking about fairly crude mushroom or "bolt" shaped plugs of course, and I've not tried it yet so it might completely suck. If it works, it would probably be easily scalable (a drill press can go through a lot of wood fast). I'll probably try it for kicks, since I've had cement and sand sitting around for a long time...
V