There was (are) a black plastic "guard" that is screwed to that opening and it has a hole for the arm to go in and out of. I would have no idea where you could come up with one. You might try the guy at Quartzite who bought a bunch of the National stuff after they sold out. I'm not sure of the name of that business but someone who goes to Quartzite will know. Your other option would be to make your own by using a flap of rubber, like a piece of a heavy rubber inner tube.
2001 National Tradewinds 7370 300 Cat
2011 Jeep Wrangler Unlimited Sport
Officially fulltiming. The Journey Begins