Erg, leeku... that word can make me cringe, lol. Just be prepared for quite a few problems that can crop up if you go the clay sculpt > plaster cast > latex cast route for making leeku.
For the plaster moldings for Kit and Shaak Ti, we had help from friends who had experience with it. Unfortunately the mold for Shaak's small leeku that I had sculpted broke and there was no time to make a replacement. Luckily the Kit tentacles were usable as stand-ins and Foxbat was nice enough to make additional latex castings from his mold as well as a cast of the big leeku eventhough we were under a huge time crunch.
http://www.deviantart.com/view/10274053/ Here are some shots of the 1st Shaak Ti costume test so you can see how those leeku turned out.