3D printing template material
Wash-away filament used for support in PLA printing is typically PVA, which is completely water soluble and may serve your purpose. It is easily 3D printed as the primary filament and attaches well to the build plate.

Many 3D printer filament suppliers will carry this type of support material. It is important to keep it in a sealed bag with desiccant as it will absorb moisture from the air, rendering it useless for printing.

One such resource is MatterHackers which prices a half-kilogram at US$45. The link provides suitably appropriate information:


  PVA (Polyvinyl Alcohol) is a water-soluble material that is often used
  as a support material, but can also be used to print independently.
  PVA supports are useful for complex designs where removing support
  material manually is difficult or impossible, but leaving the part in
  a water bath overnight will completely dissolve this material.