Is there an object that meets this description?

I’m currently planning a lesson, and for it, I’ll need an object. I have a pretty good idea what it should look like: It’s a smooth surface, about 2-4 feet across. It’s mostly flat, but it has a smooth, symmetric bulge in the middle. It should be something that masking tape will stick to, and likewise, it should not be damaged by tape being removed from it. It should be safe for children, both it from them and them from it. Ideally, it’d be thin, so it could be turned over and have a matching depression on the other side. And it should be something that I can cheaply get three or four of.

Is there any common object that matches this description? Alternately, what would be a good way to make one? I’m thinking maybe one could take a sheet of plexiglass, support it at the edges, and heat it in the center, but I’m not sure how well that would work. Would I need a form to shape it into, or would it just droop into shape on its own?

How high would the bulge be when layed flat? Would it be convex of just have a bulge?

The bulge would be a few inches high. I must be misinterpreting the other question, because I have no idea how a bulge could not be convex.

Picture a flat plain, with a single hill in the middle of it.

You can heat plexiglass in the oven and lay it over a positive mold in the shape of the bulge you want. A square frame would help push the edges down flat. You could use a negative mold but it may not sag into the depression the way you’d like. There are a lot of shops that make custom displays for retail that could make something like that easily. Instead of a simple frame a piece of plywood or similar material with a hole cut out would probably work better. You could also use that technique using a heat gun on the center part that bulges instead of heating all the plastic, it will just take a little longer to form.

Another approach, maybe not suitable is to start with a sheet of polystyrene foam, sand around the edges until you get your bulge, then paint it with a good coat of epoxy.

Probably not flat enough or cheap enough, unless you find a great deal on Craigslist, but a cymbal might work for you, with the added advantage of making a truly terrific noise if one of the kids drops it.

An old and or stolen satellite/dish antenna might be what you are looking for.

If it doesn’t have to flatten out at the edges you could try a Snow Saucer like this.

I can’t remember if the feet stick out far enough to be considered a flat surface around the edge, but the lid to those old skool turtle sandboxes might work.

Plastic garbage can lid?

A garbage can lid is usually a shallow cone, not flat with a bulge, and a satellite dish is curved everywhere. A cymbal is pretty close, but again, the part outside the bulge is conical.

Some sort of toy flying saucer is probably the closest you’re going to get.

I clearly remember Don Quixote wearing a helmet that fits the description on some illustration in the copy of Cervantes I read as a child; but I doubt you could find an actual specimen today, unless perhaps some medieval reenactor has one.

I was trying to get an accurate picture. The bulge could have been in the center with flat sides.

It sounds like you want a shield with a boss in the middle?

I guess this or thiswould be too expensive for you, but is it the sort of thing that would do?

Here’s onethat’s a bit cheaper.

Or you could buy a bossand attach it to any flat surface you like.

Hey, wait, you’re a RPG-er aren’t you? You probably know some LARPers that could lend you some for free.

Ignotus mentioned Don Quixote’s helmet, which was a barber’s shaving basin. It sounds like your description, except for the semi-circular cutout to go around the customer’s throat.

I believe this situation calls for… Papier-Mache’ Man! dunh-Dun-DUNH!!! Along with his sidekick Mucilage Boy(no sting for him)

A boss like on a shield isn’t quite right-- The transition from flat to bulge should be more gradual. This hat is about the right shape, but smooth, not with the woven straw texture.

And I thought about papier-mache, but that probably wouldn’t take well to masking tape being repeatedly applied and pulled off.

So far, plexiglass is looking like the best bet.

Let me guess … the lesson is explaining how Space-time is deformed by massive objects?

Would zillswork? They come less ornate, too.