for amateur/hobby purposes (like mine and yours) i would recommend alumilite. alumilite is a plastic casting company with a variety of casting resins. for an introduction to plastic casting, they have a "super casting kit" available with everything you need to learn, for about $70.
i'm not sure the difference between alumilite and other plastics, but alumilite is most available to consumers. their website (alumilite.com) has lots of support material and step by step instructions for various casting problems. also, they answer most email inquiries (usually referring you to a FAQ on the website).
for your project you will need a CLEAR plastic resin, and some (orange) dye. alumilite offers ALUMILITE CLEAR, which you would have to purchase seperately.
here is your bare minimum shopping list:
$5 for orange dye. $25 for 32 oz. alumilite clear. 2 lbs of rtv silicon rubber, to make the mold, might be enouph (2 X $28).
additionally you will need some superglue, some material for housing the mold while the silicone cures (cardboard base and corrugated plastic walls is good), and you probably want some modeling clay, at least for sealing the mold box during cure, if not for helping to make a 2 PART mold.
so that is a minimum of probably about $100. however, i would recommend getting the kit too, to start with and learn ($70). also, if you want your lens to be really clear and free of obscuring air bubbles, you will have to PRESSURE CAST the piece during cure. i have never done this myself, but have looked into it. alumilite sells a "pressure pot" for this process, for $185. (i am not sure if this pot is large enouph for your purposes though). i'm sure there are other alternatives for a pressure pot, but homemade ones are not safe, as pressure is dangerous. to use the pressure pot you will need (according to alumilite): "an air compressor of at least 3.5 H.P. and a 1/4"coupling/adapter to attach to your air compressor". i will estimate that to be about another $100.
so pressure casting will probably add about $300 to you project cost, unless you already have those tools.
here's the diff between pressure cast and regular cast:http://www.alumilite.com/howto.cfm?ID=18