Here's an interesting project if you'd like to make your own

http://www.miata.net/garage/swingplate.html

I actually did a little googling for you, but that's all I found. If no one's posted a link to buy the kit by the time I wake up, I'll ask my friend where he got his. Best bet would be a site that sells ricer accessories.

 

^That looks like it hinges to the bottom of the opening. What makes it pop back up when you come to a stop?

 

Hm...good question. Don't really feel like reading through the instructions. If it truly does have hinges on the bottom, I'm assuming a weak spring.

 

something like this?tilting plate