Outdoor light mounting issues

I recently purchased some new outdoor lights for our house which is about to get new paint. Two issues:

1) The lights we selected (and it appears most commonly-available lights) are designed to be mounted to a 4" round electrical box. It turns out my house has 3 1/2" rounds. It seems like an adapter of some sort should be readily available, but I have turned up nothing so far.

2) Wasps. The lights are mounted on cedar siding, the boxes are mounted flush with one of the boards, and the previous lights simply sat spanning two of the boards creating a space that proved to be an enticing wasp home. I've seen various siding mounting blocks, but they all appear to assume that your electrical box is placed directly half/half between boards (and is the appropriate 4" size).

Any guidance would be greatly appreciated.

Thanks,
Aaron

 

Can you post a pic of your fixtures?

You can also scan this PDF for different styles of fixture mounting boxes.

http://www.arlcatalog.com/Siding/siding.pdf

 

So here's the electrical box:


And here's what the fixtures look like:

 

Pcboss gave you a great link for best way to mount on an uneven surface but getting back to the basic question The light should have included an adapter. The adapter is adjustable to different size boxes then the light fastens to the adapter. In fact the instructions with the light should show the adapter.

 

The lights do come with a mounting plate, however while the plate aligns perfectly with a 4" round box, none of the holes/slots align with boxes I have.

I have seen the Arlington Products, and was looking at the the 8141DBL as the most likely contender. That said I was unsuccessful identifying where I could buy one locally and didn't want to buy online since their doesn't seem to be any documentation about what height siding it's for, i.e. are the notches aligned for 4" or 5" siding?

The other siding mounts from Arlington position the plate half on one board and half on another which would not line up with the boxes I currently have that are all on one board.

Thanks,
Aaron

 

I have used a piece of siding turned upside down to even out the taper in the siding. Or cut your own piece that has the correct profile.