Based on random outputs from droppers and different power output levels of hoppers for stackable/unstackable items, random bits are generated and stored in the output latch.
The timing part, if enabled, shuffles the random number to increase the entropy. To disable the output to update when the timer overflows, remove the connection on the black blocks.

Additional Notes

Red: output
Yellow: clock input (generate new random number)
Brown: enable/disable timer

Size: 17x6x9