How to find the right fit for these lights

I don't know where you can get the ridge pieces, but I had some loose lights. I bought some white-tipped screws [Blue Hawk 40-Count #10 x 1-in Zinc-Plated Self-Drilling Interior/Exterior Sheet Metal Screws (Item #: 123072 | Model #: 604238)]. Then I used a small drill bit (3/64" or 1/8") to bore a hole through the base of the light mount and into the wood. Then, I placed a thin, sturdy cloth over the head of the screw and used a nutdriver to secure the light to the post. I pre-drilled to avoid cracking the base or any of the adapter pieces inside. The white paint on the screw did not chip so it's barely noticeable. I only used two screws per light on opposing sides of the light. ****If you do this, be sure to pre-drill & screw at the center of the base.****

I had the same problem and had to screw the base in before attaching the lights if you get flat head screws the light becomes a even better fit