It's sometimes called a siding mounting block.

Here is a link that might be useful: siding mounting block

hatman - This would be on existing siding. When you say cut0ib and finish with a chisel do you mean cut into the bottom of the horizontal board so when done installing the new back piece it will be level on top and bottom?

mightyanvil - Thanks for the link. I saw those today at Home Depot and may try them.

greenmtn

They are called Siding Mounting Blocks - check out this link which has a several versions of them :

http://www.aifittings.com/arliprod6.htm

I have run into a similar problem. The siding blocks that the electricins put in are too small for our light backplates. My problem is the length of the backplate, not the width. For the lights mounted against the hardi plank siding, I am going to use the arlington double tall siding block face plate which will fit into the box that is already installed. The also make a MEGA block which might help you out.

The real challenge for me are lights mounted against the areas that are stoned becuase the stone face is rough and won't allow the siding block face plate to snap in flush with the box. For these situations, I am going to make face plates out of wood and mount that wood face plate to the siding block face plate (a face plate on the face plate). I will also have to put in a spacer between the 2 so that the exterior wwoden face plate comes out past the stone. The inner platsic siding block face plate will snap in flush to the box - once I trim it down a little. The outter wooden face plate will provide a flush mount for the light, that is larger than the lighting fixture back plate.

It seems like such a simple thing - but has turned out to be a royal pain in the rear.

Hope this helps.

>>This would be on existing siding. When you say cut0ib and finish with a chisel do you mean cut into the bottom of the horizontal board so when done installing the new back piece it will be level on top and bottom? The block sits flush against the sheathing underneath the siding. So to install a new trim block or replace a small one with a larger block, I have to cut the siding back to accomodate this. You say you have cedar siding -- is it clapboard or something else?

It's called "Mounting Plate".