I've been thinking of experimenting with making some rock to supplement my existing LR in a 29 GAL. I've read the threads on using Portland cement and rock salt and sand molds. All seems doable, yet I want to do this on a small scale (am an apartment dweller) so 90# bags of cement seems a bit much to make 25-30# of rock. Are there sources for smaller amounts of cement or alternatives (e.g., from hobby shops) of which I am not aware? Thanks.