I thought I'd post a quick picture of my mounts for the sonar and optical flow sensor on a 3DR Hex platform.
The sonar mount is modeling plywood cut to resemble the $10 version for DIYD. I used 3 strand servo wire for hookup.
The OFS is on a piece of acrylic that I cut and drilled.
In both cases, I wanted them between the arms for a little protection in the case of a 'bad day'.
You might also notice that I moved the forward two speed controls back to create a little less noise up front.