I'm unfamiliar with the RK37 or the FEL that it would mount. That being said, the FEL 'post' is a popular mounting position for a side-view mirror.
First I needed to locate a suitable mirror (with mounting). In my case, I found a single 'Van' type at my local auto-parts shop.
My 300CX FEL posts had pre-existing holes at the top which I made a template from and then had a local metal shop knock up a bracket that would fit those holes + the mirror mounting bracket. I also included a rubber gasket (cut up a rubber floor mat) between the post and the bracket to reduce vibration being transferred to the mirror.
I painted the steel mounting bracket JD green and the result looks like it's OEM.
Mine is mounted on the right side as we drive on the left side of the road (for when I take the 4105 'off property')