Whatever silicone rubber you use, you're going to need a suitable vacuum degassing pot to remove all the bubbles introduced during the mixing of the catalyst into the rubber.
Don't worry about it staying true to the original - this is the main justification for using silicones. Their repro accuracy is unbeatable. So dont leave any fingerprints on your original because they will appear on the mould surface.
To get down into the bottom of your lens profile, you'll probably best using a system of vacuum pouring , as well. Pull the air out of the container in which the pattern is sitting, then open a tap to let the rubber in. That's the principle.
You can make all your own kit, it's all pretty straightforward.