So you are looking for a unique marble run that fits on your small desk? This 3D Printed Marble Machine is worth a look. It is 3D printed in PLA and works as a fully functional marble run. The design is pretty simple. The center has a feed screw to bring the marbles to the top. They run through 3 different tracks to the bottom.
This 3D Printed marble machine comes with 10 steel marbles. It measures 19x17x17 cm. Backcorner3D is selling these on Etsy.