https://www.kolpin.com/en-us/shop/ac...ndlebar-mount/
I took a pair of these Kolpin handle bar mounts and drilled some holes in 'em to attach right behind the seat. Every fall (like now) I remove the rear seat and I install these. They hold my crossbow.
I would think with the correct placement they will hold your shotgun quite well over\thru the roughest terrain.
