Just a thought to consider...

Rain SKIRTS have ben used by some ultralighters and would be easier to sew. To see if this could work for you, start with a plastic trash bag. If you bought the kind with a drwastring top, you could just cut open the bottom. The skirt would actually allow some breathing that silnylon pants can't. A few hikes in the rain would allow you do decide if a rain skirt is for you, from there decide whether to keep the plastic bag or sew a silnylon skirt. This may even help you decide whether or not non-breathable pants are for you.