The extra silver part goes over the tab on the bottom front of the trailer - the tab that launches roller. It's used to attach the jetpack to the trailer in alt mode. Not really. I guess you could mod the tab on the head (I assume all molds have that one at least) to be a little longer or bigger and hold it on with that alone. Doing the nail polish trick might work to make it hold tight. That aside, you could just glue some tabs onto Prime, or even glue a tab into the jetpack slot and cut a slot into Prime to receive it.