I was considering trying to mix some resin or polyurethane with drywall mud or something like that. Would that dissipate heat well? Would it be heavy? Would it let light through? It is for something to hold a small LED bulb so I do not want to make the bulb get hotter than it needs to be.
Sounds pretty dense. And I would not expect it to dissipate heat very well. But how much heat are you expecting from this LED? Calculating that might be a good place to start.
LEDs do produce heat, and in fact heat is what kills them. They don’t get as hot as incandescent bulbs, mainly because they aren’t emitting primarily infrared like incandescents are. If this is for 120V G8 mini-wedge to MR16 adapter it’s a 2W bulb. And if you don’t sink that heat effectively, it will dim quickly and die.
Here’s a random article out of many I found describing the general efficiency and heat characteristics. If the LED is 15% efficient, then the bulb that consumes 2W of electricity will dissipate 1.7W of heat, so you’ll need to sink that heat continuously.
Do you know something to add that would work better?