I made myself some convertible mittens/fingerless gloves (but of course, can't find the pattern since my old computer fried with all my links). I figure you can make these
, and just omit the flap if you don't want it. Here is another one
Not sure how easy or not these are for you, but mine were pretty simple. I've also made several pairs of the Nicholas Fingerless Gloves
. Maybe you could just finish off the thumb, rather than leave it open? I found them easy to make too.