Molding inserts?

[Hobgob]

I was just wondering if anyone has molded inserts for their tanks before. I know you can use styrofoam and seal it after but I was thinking smoother things. Styrofoam works great for rock sculptures and caves but being able to mold something would be so much more intense! So many ideas! Any ideas would be awesome And greatly appreciated!

 

[Hobgob]

Another possibility is a sealant to go over top of my molding. That way I could just seal my molding and use what ever I want. If anyone has any ideas that would be great. I'm doing a 75 gallon and would love some ideas.