How do I build this simpler gadget?
You can build this kind of circuit relatively easily using a bidirectional shift-register like CD54HC299. If you pull DS0 high and DS7 low you can shift high values from left and low vales from right.
Use the buttons to pull either S0 or S1 high to set the shift direction.
You can drive the clock-input from both buttons but you may need to delay the clock by a few ms using a RC network.
The chip can drive up to 50mA (for all LEDs) which should be plenty for 8 LED indicators, if you don't mind them being relatively dark.