I bought the rail adaptor from Kaehny :P99 adaptor. Hey, I know those guys got a bad reputation but I'm not talking about a frame, just an adaptor. As for me, their service was correct (what more can I say ?).
The adaptor itself is made from cast metal and fits very well (note that the picture next to the adaptor at Kaehny's web site doesn't show the adaptor properly fitted. If so desired, I can mail some pictures of my P99 with the adaptor so that somebody can post them on this forum as I do not know how this works).