I have a similar helmet shelf (similar to the second link) in my enclosed trailer and it works perfectly. Never bounced out. I always put my lids in their helmet "bags" so they don't get scratched moving around inside. And I have inadvertently hit a few cattle guards at higher speeds (NOT FUN in a diesel Megacab and 16 foot dual axle enclosed trailer! FYI always reserve jumping to your gasgas, not your diesel pickup) and helmets are always on the shelf when I get home. Used it for about a year now and very pleased.
here is the one I have
http://www.rockymountainatvmc.com/p/...=rider%20cargo
I have it mounted to the front wall of the trailer where it probably gets the most up and down motion during driving.