I do - I actually had one, and it worked well. There was also a full spout that had a small tube coming out of it for pouring; it worked great, but didn't have a stopper, so you ended up carrying a stopper-cap and a pour cap on a loop of cord, just like the flat disc.

As I recall, the original Sigg stopper spout had a couple of holes down in the threads; their idea was that you unscrewed the cap a bit, and could pour fuel out one hole while using your finger on the other to control the flow rate. It would have worked great, except for all the fuel leaking out around the unscrewed edges.

I haven't seen one of the discs for years, however.