Very neat idea. I like it. Here are a couple thoughts that may be of interest. First, I use 2117 shafts (they fit the nib on slimline style nibs almost perfect). It looks like your nib may be a touch proud of the shaft? I use a section of 1916 shaft to sleeve between the 2117 shaft and the 7 mm tube. They all go together pretty smoothly with CA. Second, there is a material that might work well for you to install the broadhead to the antler. It is called All-Game and is a two part epoxy putty available from taxidermy supply houses (like Dan Chase at danchase.com). It mixes into a pliable epoxy putty and hold very well. It is a sort of light tan (antler like) color and can be sanded and/or stained. Dan Chase also carries an antler stain (potassium permanganate) that gives bleached antlers a very nice look as well.