Interesting. Freelance in props concept art? Why limit yourself to just movies? You should perhaps open up to the gaming industry too. I worked in the movie industry as a props concept artist for about 7 years until I switched to the gaming industry due to the lack of contracts. The movie industry is dead here in Montreal. We have a hard time finding props designers for the game we are working on.....
Anyways, it takes time to be recognized. It takes a few movies and good contacts in that industry. Props Masters, Production Designers and Art Directors are your key people. Once you have proven yourself, they will always call you back. Patience and good quality concept art work is key but also finding the right movies that requires your service. It's not every movie that is sci-fi so this limits a lot on the amount of contracts.
In my portfolio, you can see some process of making a prop; from a concept art, maquette to final object. Mind you that it is not necessary to make technical drawings or foamcore models like I did. In my case (having an industrial design background too), I treated my props as an industrial design job. Furthermore, the director can actually see and touch the prop (prototype). 3D is a very good tool to present a WIP prop.
Someone actually asked me about props design:viewtopic.php?f=19&t=27994