Casting Using other resins or casting materials to make junk casts or base head

Polyurethane is quite expensive and volatile, and I am not sure if I should spend that much for casts that I will cut up and sculpt over. I as also thinking of making some base heads like Jungatoy. It was really a brilliant idea to have those in stock since forming a base from scratch is time consuming(specially on the head-cap locking)on top of getting inconsistencies and symmetry problems.

I was planning on using polymer clay or stone clay to sculpt over the base. Jungatoy showed methods that won't require the finished head to be baked in the oven so it's safe for us. Boiling water~ I also acquired some DAS clay so I will be sculpting with that in the future.

I looked at the thread about Hydrocal--hope it's doable for thin walls. I was thinking of using the more common polyester or epoxy resin instead. I was thinking of only using polyurethane for the final casts. I worry though that the polyester or epoxy resins here could be too viscous to pour. I could always pour on both sides and press~ or inject it into the mold. I also saw some of you guys on the workshop sculpting over Flumo casts(which is not available here.) I am actually thinking of making plaster press-molds for the head parts when I am using DAS. I've been experimenting on a lot of materials, and I think I should stop using the industrial fix-it putties since they all give off unnecessary fumes and aren't really that fine as a finished sculpt. ahehe

What do you guys suggest I can use?