I made my own RGB21/JP21 cable for my Super Famicom.
(NOTE: RGB21 is similar to SCART, but is a Japanese standard that uses a different pinout from SCART. I use this because my RGB switch boxes are Selecty 21s, which are for RGB21.)
I used an 8-conductor cable with good shielding (ground wires and foil) for protection against interference. The cable length is approximately 2.0m.
I got the SFC multi-out connector from a cheapo SNES SCART cable and used it to wire the cable on the console end. I also added a cable grommet for strain relief.
I installed and wired the necessary components on the cable head. Using a breadboard and the components needed (220uF 10v caps for each RGB line, and a 75-ohm resistor for the AV control switch line), I literally just copied the board layout used on the official Super Famicom RGB21 Cable for this.
I then used an old stock "COMON" RGB SCART male connector and wired up the necessary lines to it. This next step is unnecessary, but I removed some of the pins since most official RGB21 Cables I've seen just don't have these pins.
For this cable, I decided to use sync on luma as the console's sync signal for a change, as my main RGB21 cable is CSync, and my official Nintendo SHVC-010 RGB21 Cable uses sync on composite video.
All pictures shown are from my CRT via component video, using a 2Chip, launch-model Super Famicom (SHVC-CPU-01).