OK, cute idea, but you’ve got some practical difficulties to work out. How are you going to keep your model erect long enough for the mold to harden? If you go the constant stimulation (of something other than the penis, because it’s stuck in a vat of cold, wet goo) route, you run the risk of orgasm before you get your mold.
Then again, if the subject’s penis is anything like mine, it’s constantly squirming around beyond its owner’s control. Even when erect.
Wouldn’t you also want to “supersize that”? 