Steps
I first created the basic box of a human body to allow me to understand the proportions as well as size of the product before adding the add-on. 
Afterwards, I added holes and the extension part for my hand to rest on. This would then have additional parts with the fillet near the shoulder to guide the way of the hand and not have too sharp of a corner by the shoulder.
After all this was done, I essentially measured or considered it with my sketch model where the sizes were kind of similar.
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