REPLY THE RUSSIAN
I found your original question. If you are going to make a production mold, it should be strong. you will want to use tooling gelcoat (it is harder than regular, and comes in black or oarnge). But first you need to make the shape that you want your part to be, and use this to create a mold from. This is called a plug. Styrene foam is good to use, because it's cheap and easily sanded or cut. Use this to approximate the desired shape, keeping in mind that you need to build it up with something to make the finished surface smooth and non pourus. Bondo works great for this (cheap and easy to work with. some epoxy fillers work better (they're less pourus) but requre specific mix ratios, are harder to sand, and are bad to get the dust on your skin or in your lungs. Bondo works good, you just need to wax it really well before you make your mold from it. Finish the bondo with about 220 grit, and apply a few coats of wax. I like using Johnson's paste wax on bondo because it fills in the pores. Apply it, then buff it off using a clean soft rag. I would do 4 coats. PVA works great, too. This plug should exactly resemle the desired shape of the part.
You need to build up the thickness of your mold, so after you have applied a good thik layer of tooling gel to the waxed plug, and the gelcoat has cured, put 1 or 2 layers of light mat on it. Use enough resin to leave the surface smooth enough to lay up your next layer. If it looks like the silent chamber's picture, you have used way too much resin, and didn't get the glass to lay down well enough (lol). After the layer of glass is cured, scuff it up a little with 80 grit to allow the next layer a mechanical bond.
Woven roving is usually the best to go with on sucessive layers, cause it forms well and it's cheap. you don't want to use it on production parts, though: it ends up being heavy and brittle. Use a bi-directional cloth.
repeat the scuffing and laying of roving untill you get a good thickness (usually 3/8" for a good mold of this type). There you have your mold.
Flanges made in the mold to create a multi- piece mold (to be able to remove the part) are usually put in when fabricating the mold.The heavy type of craft paper that has a shiny surface works good, because it is easy to cut, and can be curved slightly to make the mold pieces fit together more accurately. Position it so that the seam created will allow the part to be pulled out of the mold. Cut it so that it's edge is flush with the plug's surface, and"spot- weld" it in place with hot glue on one side only(the side opposite if the first half you make). Wax the crap out of it, and lay up one half of your mold. Remove the divider, wax the flange of the existing mold half (and the plug again, you've probably wiped some wax off in the process), and build the other mold half, using the existing flange to make the other half. Keep in mind that you need to use putty when laying up fiberglass in a hard corner, or it will create voids. The best way is to make a pie bag that you put catalised putty in, squirt it into the radiuses, and laminate on top of it before it cures, trying not to force the putty out of the corner.
Oh and one last thing. Keep in mind that your mold must have a draft angle to allow the part to come out.
This type of mold should work very well for you, and the opening for the headlamp should allow you acess to make the part inside of it.
You should make your part with 1 layer gelcoat, 1 layer fiberglass mat, and probably 2 layers of an 8-10 oz. cloth. you will be able to tell once you make a part how stiff it should be. You can also reinforce stress or mounting locations with more material in these locations. Have fun and good luck.