If you feel your shins are too long, yes trim the bottoms slightly and see if that all you need to do. It is the easier trim to start with. Thighs, if you trim from the top, you will remove the return edge, in some cases it is the only way to get them to fit, and if you are game you can try and reform a new return using a crafting iron.
Can you please post an image of you wearing the legs so we can get a better idea of how they sit with the notches trimmed.
As to the shimming of your kidney, you will need to glue a strip of ABS on the inside, to give some material to glue the outer piece you will need to start taking up the gap.
Shim equally on left and right side, and think of it as an internal cover strip to join to pieces so they sit flush. Then you need to make some ABS paste.
Use your abs trimmings and cut off and mix them in a jar with a small amount of acetate, it will melt the abs down to a paste. Apply this to the join you will see on the outside of your kidney sided where you have glued on the extensions.
When it's dry, carefully start to sand the excess off moving to a finer and finer sandpaper. Once sanded smooth polish it up and your shim is complete.
There is a thread so where on here that goes through the process with picture that will make far more sense to you.
Best of luck and post those leg images.
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