Best materials for originals to be slipcast

Hi, new guy here! I'm interested in making my own tiki mugs, and while I'm familiar with the slipcasting process, I 'd like to know if anyone has any recommendations as to what materials are best suited to makng the original piece from which I'd make the plaster slipcasting molds? Any help would be appreciated, as would any recommendations about materials that aren't suitable...someone already told me that balsa wood can't be sealed sufficiently well to not absorb water from the plaster during the mold making process.

Mahalo!

Tiki Guy