You can use a silicone, but it looks horrible. I'm sure they are working on one due to the Thunderbolts grossly undersized factory battery.
I found a great site that offers hard rubber cases for the HTC Thunderbolt w/ extended battery, I received mine and it fits like a glove. They are only charging $14.99 for their cases. Hope this is what you were looking for!
There is a guy with userid moretalktime on eBay that sells a pretty inexpensive high quality TPU case that allows for use of the kickstand too.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.