Hi @rochelle2,
Thank you for your question and welcome to the Bunnings Workshop community.
You should have a look at Easy Mix 20kg Rapid Set Concrete or Bastion 20kg Rapid Set Concrete depending on availability in your area. These are fast setting, suitably strong concrete mixes for your purpose. If you were concerned about strength, you could add some Jack 70mm x 46m Bricktor Mesh around the mould for additional reinforcing. You should also have a look at Dulux Avista Oxide 1kg White Cement Colouring, which comes in a variety of colours depending on your preference.
Please also remember all necessary personal protective equipment when working with concrete including a mask, gloves and eye protection.
Let me know if you have any further questions.
Jacob
