Hi, Rick. First, I have no idea exactly what you are doing so some more info might be helpful. Second, since you will no doubt wish to test before you try it on a big project, here are a couple of things that come to mind: 1.) Durham's Water Putty. 2.) Dow-Corning makes a whole family of Room Temperature Vulcanizing (RTV)rubber products for molding.
If you have a good industrial chemical shop that supports tool and die makers, you might stop by and ask them. If you call on the phone, you will probably get brushed off quick so visit them with samples and pick their brains.