I've posted this image before, this is the " Milan Pere " attached to my wife's 2015 M6L. It's expensive, doesn't look a lot but is well thought out. The part that is sitting just aft of the frame hinge is shaped to the contour of the frame and has a heavy duty resilient insert where it sits against the body of the frame. There's also a protrusion that sits underneath the bottom of the hinge and prevents any possibility of the structure "bouncing " of the bike when unloaded and going over rough ground.
In the photo the Childs saddle and mounting bracket was added by me, as were the coverings of the foot rests. ( I used a couple of old standard diameter handlebar grips which were cut to length )
Happy to answer any queries.