This one (Sauerkraut's Xmas present) shows how I go at it.
I turn a more cylindrical plug than usual (ie less tail shape). Pick a bigger lip (this is a pikie 3 but I've always been impresssed at how well the atom shaped lips swim a skin). Put a BIG hook where I want it, then float it and fool with movable weighting to get it to float where I want. This one is meant for the canal and to get beneath the surface so it will use extra lead to float low (sans skin). On the finished plug, only the tail weight would be through wired. The other weights I just use belly weights of the same gram weight. More often I would use a single belly weight behind the hook and a small tail weight to counteract the thick tail's buoyancy and help cast better.
The other thing to consider is the size of the skins you can get. This plug is meant for medium skins and #^&#^&#^&#^&head wanted one heavier than the one I built him last year (in which I missed the swivel with the through wire and cost him two fish
). On large skins I still use the plastic A40 but had two wires break over the last 2 years so will likely build some protos this year instead.
Likewise, on skin needles I'm still unsure whether I like more tail weight or not. Taking the tail hook off a needle makes it run more level which in turn makes depth control more difficult, but also lets you fish it more slowly. I've built a bunch of skin needles but remain undecided on what works best.
All that said, I don't think there is any such thing as a bad skin plug. The only thing I find that makes them suboptimal is tail fouling or the wrong length tail for the action of the plug (you get a hinge effect instead of a S wave). The swimmers also cast poorly which is why I'm fooling with tail weights (but don't pretend to know if they are the right answer or not).