I made these 'Lance Marshall' mitts in size large with some 2.5 layer wpb fabric a few years ago. Great instructions, easy to make, comfortable and they work well. I used them for years and they're still going. I have experimented with tyvek and wouldn't bother. I made a lobster pattern mitt of my own design with the thin white tyvek used for painter's suits and they soaked through and wore very quickly. They failed miserably for anything above a light temporary drizzle.
I recently modified the LM mitt pattern and made a pair of cuben hybrid lobsters that I have yet to test because I haven't been out in the piss with them yet. The idea with the cuben is a non breathable lobster that I would use with defeet wool duragloves to keep my hands warm as I struggle with icy hands riding in the rain when it's between freezing and 5 degrees celsius. I'm hoping these will be the right tool to help.
The Lance Marshall Pattern.....
http://www.wvi.com/~ulmyog/rain%20mitts.pdfGood Luck!