I suppose the ideal would be an entrance reducer that opens and closes
depending on the weather, and I suppose that such a thing exists.  But, even
if it does, it would be costly, bulky and and unsuited to commercial
beekeeping.

The next best thing is a reducer that the bees can remove themselves when
they are strong enough and the weather is right.  We have come up with such
an item.

We cut old cardboard into stips about 2-1/2" x 13-1/2" and fold it in half
along the long axis.  This makes a V shaped reducer that can be
slipped -crease first -- into any entrance regardless of height and which
will stay there due to the spring action of the paper.  When the bees decide
it is time, they just chew it out.

We've used masking tape too, but it tends to be hard to use.  We sometimes
use duct tape, but the bees cannot easily remove it, and it sometimes does
not stick.

This is the best so far.

http://www.internode.net/HoneyBee/