Well Matt, seems like most of us have realized that rolling around in the back of truck is no good for the trimmers. After 1 season of beating my Stihl trimmers this way I did some research and decided to buy the best trimmer rack out there.
I was very excited to finally have a secure and professional storage system for my Stihls. The first problem I encountered with my TrimmerTrap brand racks was mounting them, the bolt holes didn't line-up with metal rails of my trailer so I had to drill new holes and since I wanted to mount them on the out side of the trailer I had to fab some wood shims since my trailer's rails are the angled steel. Needless to say after a month of vibrations the wood shims cracked and I nearly lost the trimmers when the racks fell over. My solution was move the racks to the front of the trailer and WELD them on, kind of a permanent solution that won't move to my next trailer. The second problem with these racks is if you don't load them just right you will cause wear to your handle/damage to your throttle, or the unit will rotate in the rack and spill gas. BUT THE BIGGEST problem I have encountered is the security issue, rather false sense of it. Because you can padlock the trimmers when I get home from mowing I will leave the trimmers on the trailer and padlock them (this safety feature is also a pain because in order to keep the trimmer in place you must slide the padlock through the pin hole for each of the trimmers, whether you want to lock them up or not.) These padlocks then get rusted and become hard to put on. Well one morning I go out and start the truck and find my equipment had been stolen! The thief used bolt cutters on the exposed padlocks and made off with my two Stihls. Thats my true story and I'm stick(edge)ing to it.
From what I can tell it looks like your design encloses the padlock and only needs one- good job.