Seriously though, Any idea what pattern to make the leather jerkin with ?
Well, since we have no extant leather jerkins, only some written evidence of their use in neighbouring lands, anything I write now will be conjectural. The first way to make one is just making it T-tunic shaped, with sleeves the same lenght as the hauberk sleeves. Another contraption I see in reenactment these days is something like an aketon made of padded or quilted leather, although that's a thing I'm not entirely sure about - don't know any historical basis for it.
So - my opinion is - make a T-tunic out of leather, and try to make it such a size, so that it will be covered neatly by the mail. That way You either have a piece of garment in a quite a simple and possible shape that might be used back then, and even if it's too much a guess rather than proper reconstruction, You always have some under-mail protection. Better still than an aketon of fabric, which is widely spread as an under-mail device in reenactment, although we have second to nil evidence of it's use in Viking period.