Based on Building Brick Technic gears, but scaled to 750%. I've used a 24 tooth gear with 3 axle holes.

The back plate of the clock includes a screw slot for wall-hanging. 

The optional rack and wedge allows you to place the clock on a flat horizontal surface.

The clock will house a standard clock mechanism of 56x56mm and a thickness of about 16mm. The threaded shaft of the mechanism needs to protrude 13mm to keep the hands forward of the clock face. If your mechanism is more/less deep you might need to scale the centre support piece for the correct thickness.

1. Push the threaded screw of the mechanism through the front axle and secure with the nut (that came with your mechanism).
2. Push the front axle into the clock from behind until the mechanism is fully inserted.
3. Place the centre support piece on the back of the clock - mind the orientation. 
4. Push fit the rear axel into the centre support
5. Push fit the rear plate over the rear axel - ensure screw holes line up with the clock front.
6. Screw the rear plate into the clock face using 3mm screws of 16mm thread length.
7. Attach the hands of your clock and ensure they rotate without interference with each other, the clock face or optional surface rack.

I've used 10% infill and 2 walls for my print in PLA.

The clock can be grey, and I'd suggest black or red for the axles.

To print the rack on a 220mm x 220mm bed, the rack can be rotated 45 degrees to print diagonally.

Use supports on the clock face. It's optional on the other parts but advised on the wedge, as it has a hollow underside, the same as a original bricks.

The rack is for optional shelf-mount and the wedge will hold the clock upright and stop it from rolling.

The wedge is best in black, so it's not easily visible.

It is possible to stand the clock on the wedge alone, without printing the rack.