A couple more shots attached including the resulting roast. I should have said in the initial post that the cooler was built using a bathroom exhaust fan which moves 650 cubic l/h. I built a box from particle board with a raised floor which holds the exhaust fan, and a SS sieve from the local craptastic/bargain store in the top. Theres holes then cut out at the bottom to give the air somewhere to go. I give the beans a brief stir & theyre barely warm to the touch in a minute.
So, a little longer in the roaster next time (might do one or two more tomorrow for backup/comparison).
Thanks to KK for the inspiration & to russellb for the PC power supply idea. The motor for the agitator runs happily across the 12v & 5v rails & the PSU was free from a mate.