They are the 18x16x8 units... I think they're available with different fan/heat options so just pick what best suits your need.

[Link: l-com.com]

They have an aluminum back board, so I just use aluminum angle and Velcro to make mounts.  Here's a photo of how the Sonos Amp is mounted:



Edit:  I've got 5-6 of these that have been in place for 6+ years without issue.  They're still weather tight and the fans are still working (even at a property near the coast).