(originally "Cassadaga propaganda", a meaningless phrase)
A small cabinet is assembled on stage, on a sheet of perfectly clear plexiglass supported by the backs of two chairs substantial enough to hold the weight. It consists of the body and two doors; the inside is lined with black velvet. Tambourines and bells, or other manually-operated noisemakers, and perhaps a slate or two, are placed in the cabinet and the doors are closed. Immediately the bells and tambourines start making noises, and are thrown out of the top of the cabinet; and when the doors are opened, showing that the cabinet is empty, the slates have messages written on them.
The cabinet is 42" high by 36" wide and 14" deep. The exact dimensions aren't critical as long as it's obvious that nobody could get in. The key to the trick is that at the back of the cabinet there is a platform just large enough for a child (or a very small person) to sit cross-legged, and there's a well-disguised slit in the velvet. The platform is supported by invisible wires which are attached through pulleys to counterweights well above the "flies". When the assistants pick up the body of the cabinet the weights cancel out the weight of the child so that there's no hidden bulk made evident. The child simply slips his/her hand into the box and makes all the noise, throws the instruments out, and writes messages on the slates.
Obviously, the body of the cabinet must be placed on the stage, before assembly, in such a way that the audience can't get a view of its rear, and while it's being mounted the same comment applies. The child must remain absolutely motionless except when s/he has his/her hand inside the cabinet; the performer must place only a known quantity of items in the cabinet.
Invisible wires are standard equipment in the magic trade, not to mention TV and movies. They must be kept absolutely clean. As long as the front of the stage is brightly lit, and the counterweights rise and fall behind some sort of barrier, the wires will not be seen. Care will have to be taken in the initial placement of the body of the cabinet so that the supporting wires don't make too great an angle with the pulleys.
The performer can make some patter about spiritualists and how they make instruments play and get thrown around in the dark, while in contrast he'll cause it to happen under the bright stage lights. For additional effect, he could ask for a question or two from the audience that the message on the slates will answer, but it's risky, because the child might very well just plain not know the answer.
It's also possible to change the items placed in the cabinet to something else. With a bit of ingenuity and appropriate patter all sorts of tricks can be accomplished! It might even be a good idea to vary them if there are to be several performances at the same venue.