Commenting on the Double Crank Mech, friend-of-the-website John W pointed out that the push rods flap about in an uncontrolled way if they are not connected to a character of some sort.
As a way of controlling unwanted flapping I’ve added a couple of glue-on stops into the downloadable file. You’ll need to redownload the file from Double Crank page then you’ll be good to go.

Fold up and glue the two stops as triangular tubes.

You can then position them on the box top as suitable for your particular project.

Thanks for the heads-up John!