I'm a little late to the party but this is as cheap and as good as anything, I have used this method a lot since finding it on Youtube.
Fill a plastic bowl with water and add about a tablespoon of liquid detergent like Fairy liquid. Then get a tube of bathroom/kitchen silicone sealant - the cheapest you can find, mine are about a euro from the local euro shop, you can go to Poundland or B&Q it doesn't matter.
Squeeze the entire tube of it into the water, it looks like a string of sausages, then making sure you have wet hands, begin to mould it into one homogenous blob, after 5 to 10 minutes it will amalgamate and begin to stiffen up. Take it out and place onto a backing board massage to the thickness required and then press into it the part you want to duplcate, allow to dry for an hour or so, you can press it with your fingers to test hardness. Remove part once stiff and begin your lay up process.
The only things you have to watch is that it is genuine silicone that you buy, and that you do NOT use polyester resin. It works with epoxy and polyurethane resins no problem. But polyester has a reaction to silicone and the finished surface will never dry, it just remains tacky forever.
The whole process takes less time than it has taken me to type this!