The reason the Brora wristwarms are so excellent is that the bit around the fingers is stretchy and doesn't go all misshapen over time. Not elastic, I don't think, but that knitty thing you can do that makes it elastic-like? And on Brora's, that bit is doubled over. Oh, I am so not a knitter, I hope you know what I mean. I had some last year that weren't like this - they were just, as you say, tubes of yarn with thumb bits. It was super annoying when the finger bit went wonky from use.
You can do it without a pattern - just make sure you have the circumference of the widest bit of your arm they'll reach to, and do it in a rib stitch.
Re thumb hole, all you have to do is knit as a rectangle, then leave a gap in the right place when you're sewing up. If the edge isn't perfect, you can double it over for a really thin hem and sew it down, so it looks more meant.