unmounted conga skins

Hey guy's: Can anybody tell me what size would an unmounted skin (flat) be for a 11.5 conga and for the tumba 12,5. Thank's.
The World of Percussions
http://www.mycongaplace.com/forum/eng/
http://www.mycongaplace.com/forum/eng/viewtopic.php?f=14&t=6146
It's easier to have more to work with than working with just enough. I like to have more.