I make all my holdfasts at TechShop.
Step 1: Aquire
Mild steel round bar
1/4" plate steel
Oxide patina finish
The bar stock you will start with should be 1/8" smaller than the hole it will fit into. My bench has 3/4" holes so I'm using 5/8" stock. The general formula for measuring how much you will use is to multiply the desired reach of the tool by 1.5 then add the thickness of the bench top it will mount in, then add the maximum thickness of what you would want to be able to hold with it plus a few more inches as a margin of error. The pictured holdfast has a 7" reach, mounts in a 5" thick top and maxes-out at 6" so I started with a bar 22" long. Some holdfasts have a more extreme curve and some make a sharp bend at nearly 90 degrees and will require more or less length.
Mild steel or ductile forged iron are best since the holdfast will need to bend and spring slightly. Do not use brass, cast iron, aluminum, or stainless steel a they are either too brittle or too soft, or both.