RainDance:
I just tried the Make your own Dildo kit from body double and had several problems... The results are in my profile picture.
The kit claims to work for ANY size penis, but the cup they provide is only 8" tall... Like a good boy-scout I made do and jury rigged an extension...
Next, I mixed the 'quick-form moulding gel' per instructions, but by the time I put myself in it it had already began to set. I didn't have the foresight to take a Viagra like some others have... The gel was hard enough and I was soft enough that I twisted in the cup (though it's not perfectly straight it's not a corkscrew!)
So now I had a mold (I didn't at this time fully realize the quality of the mold). I mixed the two bottles and poured it into the mold; the instructions recommended keeping part of the latex to make a scrotum, but by eyeing it I figured I would need all of the latex for just my cock. I emptied the mix into the mold and, much to my chagrin, it was about three-quarters full!
Feeling ingenious and having a little time (the latex sets overnight), and feeling that things couldn't get any worse, I wrapped a candle in some gauze to increase it's thickness, slapped a condom on it, and pushed it into the latex to displace some volume and thus fill the mold. I figured I could put one of those little 6" "muscle massagers" they sell at Spencer gifts in the hollow center after I pulled out the candle. I did my best to make sure the candle didn't touch the walls of the mold. I failed, as evidenced by the hole at the base, because of the curved path it had to pass through.
All in all, a waste of time and money. The idea of a plaster 'master' mold may have some merit though...