I tried two ways, that both work quite well.
If the light is incorporated in the kit, the above technigue applies. You can use a drop of transparent resin or epoxy glue, too. Be careful, some glues may react with (read MELT) the plastic of your kit ! Try them on a piece of sprue first ! Instead of aluminium foil, I use mylar, the fancy shiny metal gift-wrapping stuff
If the light comes separately (Most AFV's...), you can copy the kit's light with transparent resin. The normal procedures for resin copying apply, but you use transparent resin instead of the traditional PU resin.
The result is a perfect copy of the original part, but transparent.
Even if the lights are incorporated in another part of the kit, you can do this with the whole kit part and paint the transparent copy as if it would be a normal kit part. Most smaller kit parts can be easily copied.
This gives by far the best possible results. You stick a piece of mylar or alu foil behind the transparent part, and you have superb headlights.
There are two drawbacks to this procedure.
1° cost. If you are not familiar with pooring resin and not equiped for it, it would not be worth investing in all that just to make better headlights. Even if you are equiped for it, you need transparent resin (see post on transparent resins in this forum). This often comes in 500 ml jars, a bit over the top just to make two itsy pitsy headlights...
2* time. small transparent resin parts take at least a week - if not a month - to harden completely. You NEED this patience, or you will end up with a sticky part and a dirty mold and you'll have to start all over again.
If messing around with resins, silicones and new techniques are the things that get you going, you WILL be happy with this technique !
The headlights on the M35A2 below are cast in PU resin with a piece of mylar in the back for reflection.
