You can make moulds from .060 sheet styrene, most of the ones that I've made are two piece moulds. I use cooking oil, wiped on sparingly with a paper towel, as a mould release. For the actual pier or abutment, I use a product, available at places like Home Depot, called Durabond 90. It's a patching plaster that you mix with water to whatever consistency you require, and is available in anything from 2 lb. boxes to 33 lb. bags. In about 90 minutes it will set and the mould can be removed. Depending on the size of the casting, it may take a couple of days to completely harden, and I do mean harden. If you wish to carve in stone detail, etc., do it before it gets too hard. It can be filed, drilled, or cut with a saw, but it will dull tools quickly. I have cast bridge piers and abutments in HO scale from 10' to 50' high, some with integral wings and with stepped or notched tops. When I require several similar style piers of varying heights, I make a mould for only the tallest. By designing the mould to stand upside down while being filled, the shorter piers can be cast in the same mould by filling it only to the required height. The finished castings can be stained or painted: I use a very thin wash of Polly Scale (a few drops in a cup of water, applied with a brush. When this dries, I use an airbrush to add rust stains and high water marks. Hope this is of some assistance.