Custom 3-D Background

[Minkness]

Hey all. I have done my own 3-D bg before but want to do another one as a project. My idea was to do a Groot bust growing out of a tree, and using the top of his head as the area that holds food or water dishes.

The way I'd like to do this is to set it on styrofoam then use sculpy clay for the detailed bust. It would be kind of a shell as I wouldn't want to make it too heavy for the bg to support once I use silicon to 'glue' it in place.

My question is this. Since I want to make it pretty detailed I'd like to use paint. I was hoping to use acrylics since they are a bit plasticy and seal well without getting too much water damage if they get a little wet. The enclosure is for a crested gecko so it will be mildly humid.

If acrylics are not a good choice what would be a good substitute?