It is a gas cap access cover. I got the full trailer cover, since I have no garage. It is actually tight as a glove and very well made. It has 2 underbody straps and a perimeter strap, and you can really get it snug. I can't see riding on the highway on a trailered SS.Is that a gas-cap access port I see in the cover? Given the costs of the Polaris covers and the relatively loose fit for what they call a trailer cover, I think I'll go with the #2 cover from Oreilly.
The dust cover fits tight. The rear covers the tire completely too. Does your full cover as well?
Someone else posted pictures of the trailer cover to show where it had worn thru in the area of the mirrors. I believe he actually thought that the trailer cover could be used with the Slingshot on a trailer as described on the product webpage.