This method is rather messy but I found it to be the best way to create a comfortable handle for my weapon. If you are going to spend hours holding your prop, you need something that has the shape and size of your grip. Anything larger or smaller and your fingers and joints are going to hurt for a few days.
First, make a loose fist and measure the distance between your thumb knuckle and the proximal phalanx (look it up!) of your index finger. That will be the width of the handle. Then measure the distance between one side of your hand to the other, and add 1-2 cm. That will be the length of the handle. Now draw a rectangle! I made this on a 10mm EVA foam.
Draw the rectangle twice on the foam and cut both pieces out. Now it's time to calculate and cut the depth of the handle. I took the design of the rectangle and subtracted 2cm from it. The reason behind it is that the depth had to be the exact size as the width. I removed 2cm from the pattern because of the 10mm thickness of both foam pieces. I draw that twice on the foam and cut it out. I only glued one of the depth pieces between the 2 width pieces (first picture). The 4th piece was temporarily attached with paper tape on the inside. I needed it to be stable but not permanently glued. Then I held it the way I would hold my weapon and traced the layout of my hand.
Now the messy part begins. I started carving and sanding down the shape of my hand with a rotary tool. Once the shape was comfortable in my hand, I removed the 4th piece (taped temporarily onto the handle). I put contact adhesive on the outside of the foam pieces, both the big (3 parts) piece and the individual 4th piece. I put adhesive on the back of a faux leather piece. After it dried I carefully covered the foam pieces with the leather.
Then I glued the big piece on the bow, with contact adhesive, and lastly added the 4th part (also covered in faux leather) onto the gap of the handle. Unfortunately, due to potato camera, the final picture isn't clear enough to show the handle.