Here are the parts needed to build the boot switch setup. They are, heatsrink tubing; DPDT switch; and two LEDs and connecting wire from an old case.
Remove the wire and connector from the plastic cover of the LEDs. Cover the connectors with the insulating heatsrink tubing. Remove the LEDs from the wires. Two of these are needed, one for each drive.
Solder the wires to the switch. Notice the stager of the wires. This way only one set of wires will be connected at a time.
Attach the switch in a convenient location, and attach one set of wires to each drive. On the drives, put the connectors on the jumpers that will set the drive as device one.
The drives in the machine are in the lower front of the case. I used a 3.5 inch cage from on old case.
Now the drives are right in front of the input fan.
Here you see the Western Digital as device zero, and becomes the boot drive.
This shot shows the Segate as the boot device, device zero.