I'm with Keygrip in calling thses checks 'pre-landing' rather than 'downwind' as my home base only has a left or right base join (depending upon runway in use)
Re. giving a decent >10 sec carb heat check, I pause at the point of selecting 'hot', having stated out loud the rpm setting beforehand and give a running commentary on having a REALLY good look round for other traffic, restoring to 'cold' and 'revs returned to original value' just before turning/joining base.
With our small circuit at Denham, you've got to work quite hard to get everything in - call 'downwind' as soon as the wings are level, turn base as soon as the carb heat check complete.
Cheers,
TheOddOne