Either will work fine. If you use a round box just make sure your light fixture base is large enough to cover the round box cutout.

 

Arlington www.aifittings.com has some very nice products to help with both fixture and receptacle mounting in many types of siding materials.

 

Can one of the blue, plastic, round carlon boxes be used or should it be a metal box?? The blue plastic box doesn't seem very sturdy for an outside fixture subject to wind and whatever else. Although, I guess the fixture would mostly be supported by by the j-block being fastened to the wall.

 

Get a mounting block with the integrated space for the wiring::

 

or this

 

Both blocks above are made to fit over Double Lap siding without having to remove the siding. You could of course buy the blocks that install by removing a piece of siding, cutting the siding, securing the block and reinstalling the siding.

 

So I bought some J-blocks made by Arlington that will be installed under the siding and then cut the siding to fit. Are there tips or advice anyone can offer for cutting the siding? Using tin snips, jig saw, razor knife???

 

Leave a 3/8" gap between the siding and the siding block all the way around it or the siding will have no room to move.
I use my ossilating saw to cut it out, a pair of heavy duty snips will also work.

Anyone else notice one of those flush mount siding blocks was being installed wrong that someone posted a picture of? Someone drilled a hole in the center of the lap instead of right at the lap.

 

as long as the ends of the panels are free expansion should never pose a problem,water infiltration could be though