Tooling boards are - unfortunately - very expensive for a hobbyist like me, especially if thicker and larger pieces are required. Therefore I wonder if there are alternative materials that have been used by other forum members instead of proper tooling boards to keep the cost low.

The obvious possibility is MDF but I'm not sure if there's a way to get the surface good enough. Does anyone have any suggestions?

My plan is to cnc mill the mould directly without making a plug first, but I could change plans for specific reasons.

Kind regards
Martin

What kind of temperatures are you looking at curing? As my tooling board is good up to 70degrees. 
You won't find any recipes for tooling block as the suppliers add a lot into the formulation.
I'm working on a project trying to reuse or recycle tooling block as so much is used by the composites industry and 99percent ends up I
 in landfill.
Aluminium billetsare almost comparable in cost to tooling board now. So I'm using more of it.