A few weeks ago I ordered some "Instant Mold" from the Coolminiornot shop. This product is a clear waxy plastic, which melts to a soft consistency in hot water and can be used to make press molds of somewhat simple items.
There are a number of demo and review videos out there, so I didn't bother doing one. But I did a quick test that might be informative. Space Marine shoulder pads are a popular target for this, so I tried out a couple for my test run.
The pieces I'm duplicating: an old SM medic and a random Terminator arm.
The softened Instant Mold formed around the parts without any difficulty, and appeared to hold a very fine detail level.
The results. I'm pretty impressed! There's some loss of detail on sides of the Medic pad. There appeared to be some air bubbles in the Terminator seal, but it looks fine out of the mold. I imagine after a few more tries I'll get a more consistent result.
One thing: some forums I've seen have pointed out that polycaprolactone is available from various sources for cheaper than IM's price. That might be worth pursuing, but since this stuff is reusable, I'm not going to worry about it.