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To make somersaults we need foil from several bars of chocolate. You can take the culinary foil, but then do not eat chocolate. We will also need an old computer mouse. You can do without it. The fact is that the only reason we need it is a ball. If you already have a ball, then you do not need a mouse.
All manufacturing technology, I think, is clear from the photos.

Remove the rubber shell.

Instead of "mouse" you can take any other ball.

Wrap the ball in foil.

We give the form of a cylinder.

To get the cylinder right, you need to shake it up and down. The ball moving inside the cylinder will straighten it to the desired shape. We wrapped the threads just in case, so that the ball does not tear out the ends of the cylinder during shaking

Align the shapes slightly. Thread can be removed

The inclined plane can be made of cardboard. For young children, you need to either take a wide inclined plane, or make side walls at an inclined plane. Otherwise, the toy will fall abroad all the time.
But for office entertainment, the usual wooden ruler is suitable. On a plastic ruler the toy will slip and somersaults will not work.
The task is simple - by adjusting the slope of the ruler to reach the finish line and prevent the toy from falling overboard.

The second varinat tumbled:

Somersault 2

This fun toy will appeal to kids. She slowly descends along an inclined plane, while doing somersaults (before each somersault - a short stop).
Paper somersault. She glues on a wooden disc. (Styrofoam can be used as a blank.)
Take a block of 25 by 25 by 75 millimeters, round off the edges, as shown in the figure, clean the blank with sandpaper. Having put a layer of paper on the blank, glue the second one on top, then again and again so that a wide paper "rim" with a thickness of about 2 millimeters is formed. Remove the disc, glue two discs (from cardboard) to the rim. Insert a metal ball with a diameter of about 20 millimeters in advance. Fix the sides properly so that they do not fall off (you can use strips of paper). It remains to put the somersault on an inclined plane.

Third option also from paper, but without any blanks. Size is approximate. Everything will depend on the diameter of the ball you can get.