Most two-part putties will hold just as fine a detail as anything else; just look at round figures. SOme are better than others. I use Aves Apoxie Sculpt on sheet plastic (I have lots of it) and I had considered using thin sheet brass or aluminum, but I found that the plastic is easier to cut with small straight and curved scissors. I use .015 or .020 thick plastic. Thinner plastic will curl as the putty cures; thicker putty is more difficult to cut. Cured Aves will carve, cut, sand, and drill easily enough. I usually sculpt small sections at a time starting at the top, then part of the mid-section, then something on the bottom. That way I'm not dragging fingers or tools through already sculpted, but uncured putty. Once it is cured, I can sand areas smooth and carve in additional and finer details before I add additional layers and start the process over again.
I have used Sculpy once on a Cheech Wizard cartoon piece from the '70s. I was put on glass, sculpted in one go, and baked on the glass. It was easily released from the glass without using any release agents. I understand fresh Sculpy can be added to already baked Sculpy using a Sculpy adhesive, but I haven't tried this. Do not use Sculpy on sheet plastic and bake it. Bad things happen...
Do go through Mark's post about Gary Dombrowski's pieces. Great stuff!
If you look at the Brian's pics, you can see the complexity of the engraving - in reverse of the actual casting. Tricky that; it requires planning and attention to detail. Also note the air channels that allow air to get out as the metal is poured in. Hard vulcanized rubber used in spin casting also have these. Someone here was trying to order slate blocks from Germany and have them shipped to the US, but it wasn't cheap. I don't know what became of that.
Slainte,
Glen