I have made silicone molds, too, but it is a lot of work.
And I couldn't get the fine detail I wanted, or use soft masters.
I had really good luck using Solarez UV cured polyester resin.
Here's what I use:
http://solarez.com/products/low-voc-dual-cure-polyester-resin/
I bought a quart, and it lasted a year, mainly for top coating hard baits.
Once I had used it to repair the broken nose on my son's surfboard, I realized it could be used for mold making, too.
I made some fine finned molds using factory soft plastics with fine fins as masters, and it copied all the details, Just be sure that you use a slick base plate to glue your masters down to, and seal the four sides down real well, too. The resin is viscous, and will find any crack or hole and run out.
I didn't use a release agent when I made my mold. It would probably help in getting the mold off the base plate, but I was afraid it would interfere with getting the resin to fill all the detail.
I added the resin inside the garage, and then took it out into the sun to cure. I did it in layers, so it would cure completely.
I brushed on the first coat, so all the details got filled really well, set that for a minute in the sun to begin curing, brushed on a second coat, another minute in the sun, and then filled the mold box so I had at least 1/4" over the tops of the masters.
I left it in the sun for half an hour to fully cure, and then took the sides off and pried it off the base.
It was strong and rigid.
Because the poured mold wasn't perfectly smooth on the side I poured from, I used a belt sander to flatten it, so it would lay down evenly when I poured my plastics.
I had some flippers that got fully covered, so I drilled some tiny vent holes down to each one, to help them fill completely, and they do.
I use spray pam as a release agent on the mold when I pour my plastics, and it works great.
I don't think it will work with hard masters.