How about a large escutcheon or plate which would straddle a larger area and take away attention from the line made by the ribs? For example, look at the plate behind this meter which is painted the same color as the siding. If you had a plate like that under each light fixture you wouldn’t notice where the lines in the siding were and it would provide a flat surface to mount your light. If you used a round plate you could paint it the same color as the light fixture and that might look nice: 
For example you could add a large round black escutcheon on the wall under a fixture like this and it would complement the style of the fixture: [IMG2=JSON]{“data-align”:“none”,“data-size”:“full”,“height”:“212”,“width”:“266”,“src”:"https://images-na.ssl-images-amazon.com/images/I/31ixPZvd62L.AC_SY400.jpg)
Here. I found a site which sells the plates and has lots of photos for ideas. They are called mount blocks:
https://www.kaycan.com/mount-blocks-utility-vents/