I've had the same problem with a Yakima box (located as far back on the rails as reasonable - similar to your install). My temporary fix is to unload box and lift with a lot of effort. I looked through the catalog that you linked and couldn't find a strut with the locking collar. It may be possible to mount one or two additional struts (paralleling the original(s) or just replace the non-locking side with a higher pressure strut.