Clip or something to keep hatch seal from slipping off?

I feel my Google Fu is failing me badly, looking for some help.

On my shell (Flip-Pac actually) along the bottom of the door/hatch at the back there is the typical rubber or vinyl seal that overlaps the tailgate to keep water and dust out. It is attached to the door by having been slid into a mating groove in the bottom door.

Seal looks something like this:

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The little arrow shaped bit hanging off the bottom is what would slide into a matching groove along the bottom of the door. (This is not my actual door seal, but it is representative of what mine looks like as far attaching to the door)

Problem is that after driving for a good while, especially off road, it slowly slides out one side or another a few feet and is left dangling in the wind back there. The seal extends past the edges of the door by an inch or so on each side and it really seems there should be some sort of clip one could crimp onto the little arrow portion at each end to prevent it from sliding back through the groove on the door. But I can’t find anything like that with any sort of search…

Feeling stupid - what exactly is suppose to prevent these kinds of door seals from not working themselves off in one direction or the other?