Ram,

Yeah, it's totally normal to do a through hole thermistor for the heat bed. But like Roxy says, they're usually attached via Kapton tape, which is probably a little easier to deal with than Silcone. I like the sponge idea too.

http://roaddoglabs.com/wiki/images/t...assembly_2.jpg
http://www.ultibots.com/epcos-100k-thermistor-1-pack/