Miniature tension rod?
If the gap is such that one could insert a mechanism into the space, a rock climbing cam may be an answer. The item is sold in sizes based on the width of the crack in the rock to be bridged/wedged. They are astonishingly expensive, but considering that one's life is hanging from this device, perhaps not so much.

A quick search shows some priced as low as US$20 and a few in the US$40 range, but most are outtasight for the aforementioned reasons.

You'd be able to insert the cam into the gap, tug it into place, spreading the load bearing segments and attach substantial weight to the fixture. When done, remove the decoration and push or tap upward to release the cams.

If you live near rock climbing regions, an outfitter shop would have stock. You could ask if anyone has a worn or otherwise useless device, as you would not be putting your life on that line.