Sorry I've been away so long but I've taken over the duties of Zookeeper/Admin at www.coreplanets.com
(yeah, shameless plug, I know).
Anyways, I just had to let you all in on a new source material I've been using now that allows me to make great molds and it costs almost NOTHING hardly! What is it you ask!? None other than Hot Melt Glue! All you need to do is create a wall around your part you want to cast (your master). I do this with lego blocks on a lego base. Now, I use a substance I found at walmart called 'Goo Gone Extreme' (its about 3-4 bucks for a very large container) and I douse the inside of my walls and the flooring with this stuff. I also put this on my master as well. What this stuff does is prevents the glue from actually sticking and bonding to the legos and master. Hot melt glue is VERY potent in terms of sticking, mainly because hot melt glue is really liquid plastic and vinyl.
You'll need a glue gun for the next step, and these can range from 2 bucks to over 50 bucks for a really good one. I have an el cheapo $1.97 pink glue gun I grabbed at wm and it works just fine. I use high temp glue sticks (which I got a bag of these for 1.50 at wm too) and fill the 'box' you just made up. Now, for bigger boxes, I suggest getting a bigger gun that will take a bigger glue stick. Mainly because the short ones harden up faster and you have to constantly reload.
Now, you do just like you would in other molding situations and put the master about half way into the hot glue (REMEMBER TO DOUSE YOUR MASTER WITH THE GOO GONE!). Put some tiny balls or something around the master as well halfway in so you have a locking device. What I do is I have some of those mardi gras beaded necklaces that people throw in parades. You can get them ultra cheap too. I just tear off the beads and they are the perfect size.
Now wait till the glue has hardened back into the semi rubbery plastic form and douse the master, box, and top of the glue again with the goo gone. Oh, and pull the beads out too. You are done with them.
Now, shoot more glue on top of the master and glue mold. Wait till it hardens, then seperate the two halves. There, you have your mold! If things are not coming apart, you need to use more goo gone. Trust me, the hot glue will bond to your items without this stuff! Its a real b*tch to get off too!