I used this method to clone two exterior end caps from a profile that runs down the side of our RV. Really impressed how this turned out considering the end caps are quite complex shapes.
BEFORE STARTING THIS, MAKE SURE THE PART TO BE CLONED IS A SHAPE THAT CAN BE TAKEN OUT OF THE MOULD. IF NOT LOOK ON THE NET TO SEE HOW TO MAKE A TWO PART MOULD! For example an egg timer shape (figure of eight) would need a two part mould. If in doubt, post an image of the part on here and I'll tell you if it can be done using the method below.
Flat piece of plastic - I used a CD case.
RTV Silicone Moulding Rubber - DWR Plastics, ebay $10 for 500g
Fast cast resin - part A & B- DWR Plastics, ebay $12 for 250g A&B which makes 500g of finished product
Plastic disposable cups
Something to mix with - I used an old Costco club card cut into strips.
Wash the part to be cloned and dry thoroughly.
Take a small piece of blue tack and roll out into a long thin worm.
Place blue tack around the bottom of the part to be cloned and push the part down onto the plastic until the blue tack starts to squeeze out from the underneath.
Trim the excess blue tack with the craft knife. The blue tack stops the silicone from going under the part and keeps the part in place while pouring the silicone into the mould.
Build a Lego box that goes all the way around the part to be cloned and stands about 1cm above the part.
Roll out another blue tack worm and go around the underside of the Lego box and then push the box down onto the plastic, making sure your cloned part is central within the box.
Trim the blue tack from inside the bottom of the box.
Spray all over the inside of the Lego box and part to be cloned with silicone spray. Use the small art brush to even out the spray and smooth over any drips (these will show up on the surface of your new parts if you don't do this)
Mix the silicone rubber and hardener as directed (mix well, including from the sides of the cup)
Pour into your Lego box in one continuous stream. This will help to get rid of any bubbles.
Leave overnight to set
Take the Lego sides away from the mould (that's why we use Lego, it's easier this way!!)
Carefully take the original part out
Clean up any edges on the mould if needed.
Spray the inside of the mould with silicone spray the turn upside down to drain the excess (use the art brush to smooth over the inside of the mould if you can get it in)
Mix part a & B of the resin as directed in a plastic cup and pour into the mould (again in one steady stream). Make sure you are on a level surface.
Take off any exceeds from the top and leave the mould overnight.