Yeah, customizing the riveted stuff is a pain, because of all those darn rivets!
In terms of adjusting the fit, one of the biggest benefits comes from removing some material from the sleeves so that they taper towards the elbow. You need to leave enough width there that you can still flex it, but make a smooth transition from the width at the shoulder down to a thinner width at the elbow. If the sleeve is longer than the elbow, you can make the width even from the elbow to the end, but you may also want to add an expansion area (gusset?) around the back of the elbow to aid in flexing too.
Adjusting the fit of the body is more difficult, since technically the back should be slightly wider than the chest (allowing the sleeves to bend forward more easily), and it should widen slightly at the shoulders. The shoulders also shouldn't be perfectly horizontal at the top, but angle up towards the neck opening slightly, since that's the natural shape of your body. Also, at the bottom of the hauberk, it should widen as it gets closer to the knees, away from your hips. This latter one is one of the easier to do, since it can just entail adding triangular expansions to the sides of the bottom of the hauberk.
These are all sorts of things that are often overlooked in off-the-shelf armor.
