yes i have im not sure what you are trying to do but , out of experiance the material that you use is kinda tricky to use! the molding material is very fast setting !!! so in order to combat this you must mix with COLD water! then after that the stuff used for the product is pretty simple and like a day or 2 later you have a new toy------- however what they fail to tell you is that this material is rather hard and duplicates your skin wrinkles and texture. remem i said its a hard material so if you plan to actually use it you need a condom or you are going to get tore up!!!!!!!!!!!!!!
Does it not come with some sort of releasing agent, or could you use KY to stop it sticking?
I can't help feeng wearing a rubber will spoil the result a little.
My partner and I did something similar, We made a mold of our hands with the plaster of paris, Use a lube. Butter worked for us. make sure that it wont break down the material though of what ever your casting. and dont use a condom. it will have creasing in the mold after. ( assuming you only want your wrinkles and creases)
If you're going to do it on your own, instead of getting a kit that's made for molding body parts, do some research to ensure you use the right kind of casting agent. Some of them are bad for the skin or heat up when activated to such a degree that they actually can burn you. I believe real plaster of paris is one such no-no, and you're supposed to use an algae-based casting material instead for safety.
You may want to read up on making molds from body parts and talk to the folks at your local mask/special effects shop or similar experts on the different materials you can use and best way to do it. You can, of course, tell them you want to make a life-like replica of your hand or whatever, instead of your cock. If you use the right products, you could potentially make a latex, silicone or acrylic dildo, instead of being stuck with what comes in the DIY cock-molding kits. If the dildo is a porous material, you will need to use a condom over it anyway, as it can't truly be sanitized. Other materials, such as silicone and hard plastics, can be sanitized, however, so you may want to take that into consideration when you're choosing your material(s) (if you make a good mold, and use the right releasing agent, you could likely make several different dildos out of it).
Getting hard and using a cockring is a good idea regardless, provided you don't leave it on longer than 20mins or so.
Click me for some info on making molds of body parts.