I am trying to install 6 new wall lights. The old lights had a standard round bracket, but the new lights have a rectangular bracket (see photo) with holes that are not really designed to line up with anything -- they are designed to just screw into a wall. This is a small box - 3" between the screws and the box is slightly inset on the wood wall. I can't find any crossbar or plate that would work. I'm thinking I have to cut a round piece of 1/8" plywood -- but then the junction box is covered with wood, which doesn't seem safe. Looking for ideas
Extenders for electrical boxes are typically called "mud rings" or "extender rings". Do a search and you'll find an amazing number of sizes and shapes, both metallic and non-metallic.
Example only of one type:
Another option is a small plastic bushing that fits around each screw that holds the fixture/bracket to the junction box and acts as a spacer between fixture and box so the screw is secure. You can stack them for your specific gap thickness. Example only:
See also this previous question: how-to-attach-a-wall-plate-to-an-old-junction-box-with-different-screw-hole-spac