Intro: Custom Steel Cable Crimp
Steel cables can be spliced in a variety of ways -- crimp, weld, solder, glue, braid, etc. Here I'll use a short length of copper tubing from my scrap metal pile to create an end loop on a 3/16" (4mm) cable. Crimp kits start around $3 at big box stores, while ferrules generally cost more and use a special tool to compress. Scrap metal is often free! This copper tube appears to be from an old gas fixture, but often you'll find them as water supply lines for sinks. You could also theoretically use solid copper electrical wire (no sheathing) to create a crimp with solder, but I won't illustrate that here.
If you have many crimps to make and no special tools, copper tubing is perhaps the cheapest option.
Step 1: Get Your Cable, Find Some Tubing
Cut the copper tubing. Use a pipe scoring/cutting wheel if you can. If you make a ragged cut with metal shears (as I've done here), flare open the end with a tool, like a screwdriver. Hammer the tubing slightly flatter, so that two diameters of steel cable can pass through it. This particular bit of copper tubing works perfectly for this cable. You can also use brass, aluminum, steel, etc. Brass and copper are probably the most forgiving for this purpose.
Slide the tube down onto the standing end of the cable.
Step 2: Flatten the Crimp
Pass the working end of the cable back on itself into the tube, forming a loop. If you have some gorilla glue or similar adhesive handy, get some drops into the crimp and along the mating surfaces of the cable. (I didn't have any handy, and it's not essential.) Hammer the tube flat, capturing the cables inside. Don't hammer too much -- the copper will expand and become loose, or eventually crack.
Step 3: Indent the Crimp
Use a tool, such as a flathead screwdriver (with a truly flat tip, not sharp -- a sharp tip will cut the copper) to hammer indentations into the crimp, lengthwise down the valley between the two runs of cable, and perpendicular across the width of the copper. These key the crimp into the cable.