You could probably go on the mother site, (http://www.lurepartsonline.com/Online-Store/Lure-Lips/Lexan-Lips-Rounded-Style.html) and order a few different ones to see what would work best for yourself? Just an idea. Looks like they sell lexan lips up to 1/8". I'll have to double check at home what thickness I used. I'll let you know. For my 6" lures, I think its around 1/8".
Another good option which I do now is track down a local glass/window shop, or plastics manufacturer (this may not work for you obviously depending where you live). I found a shop in my community that does windows, they often deal with lexan for multiple different applications. The guys let me have a bunch of different thickness little scraps that were completely unusable for them. But, I could easily get a bunch of lips out of each piece. They are now my official lexan supplier, (it helps that a few of them fish, so I repay the favor once in a while).