According to AeroMarine Products' page on moldmaking, (emphasis from source)
Generally, silicone RTV mold making rubber does not stick to anything, and nothing will stick to it. The exception is that it will stick to itself, other silicones, silica, and glass.
In the past, I have used cut-down and clean milk cartons, which make an effective base for a square mold. YouTube resin caster Rybonator has used single-use plastic cups, paper cups, plastic food containers, and custom-built constructs made of stiff cardboard to produce molds. I've additionally seen multiple cases of people building their mold box out of Lego blocks, an example of which can be seen here.
The key points you'll want to consider are:
- Avoid any material to which resin will stick (no glass, silicone, or other silicates).
- Avoid anything that will later be used for food. If you aren't using food-safe silicone, it's best to err on the side of caution and keep your casting materials separate from anything you'll use for preparing or serving food.