Probably best with a push entity, if they exsist in tf2. Either trigger or func_push so that when a player comes close to it they fly upwards again. Not sure if it works in tf2 though, and there are probably other options as well.
Try the actually trigger texture, toolstrigger, and make sure you check the Client box on the Flags tab. Also, upward pushes won't effect the player unless they aren't touching the ground.
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