Using an electric jigsaw, cut out two boards to be the chassis and the top. 16cm by 11 cm is a reasonable size
Place together and drill 3 holes to.
Separate chassis and top using wall plugs. Screw the top to the bottom to check they are in the right place.
Screws from recycled electronics gear are cheapest. Self tappers work fine as well. Make some brackets to mount the motors, use 2.5m 25mm nuts and bolts.